Macduff of the death of this wife and children. This only heightens Macduff's need to avenge his family's murder.

It's Macbeth's resolve that's weakened when Macduff tells him that he was "from his mother's womb / Untimely ripp'd" (5.8.19–20), and Macduff, "being of no woman born," kills Macbeth.

As Macbeth descends into insanity he becomes utterly obsessed with eliminating any threats to his power.  In determining which threats to heed Macbeth becomes enthralled with all of the ideas that the witches give to him, believing that all of the their prophetic warnings will come true.  It is clear that Macbeth seeks to eliminate threats to his own power due to the fact that he first murders Banquo and his children in order to ensure that Banquo's children will not pose a threat to Macbeth's rule (or that of his familial line).  Moreover, in heeding the witches prophecy (who warn him of Macduff), Macbeth is required to take action against Macduff.  When the murderers arrive to Lady Macduff's hiding place, they seek to find Macduff himself, perhaps not fully aware that he has fled, and instead find the family there.  They kill the family in part to eliminate the threat to Macbeth, but also to send the message to Macduff that they will not fear him and that they believe him to be a traitor.  Ironically, it is in this moment that Macbeth seals his own fate.  By murdering Macduff's family he ensures that Macduff will retaliate and instigate an insurgency against him, ultimately concluding in Macbeth's downfall.  This is the final act in the course of events that guarantees Macbeth's eventual death.  

Macbeth is sinking deeper into insanity. He is all consumed with doing whatever it takes to keep his crown. After he sees the witches again, he fears that Macduff poses the biggest threat to him. The witches warn Macbeth.

"Beware Macduff, beware the Thane of fire. The power of man, for none born of woman, shall harm Macbeth"

This frightens Macbeth so he wants to get rid of the threat. Macduff is in England helping Malcolm build an army. Malcolm is the son of Duncan, who Macbeth killed. Macbeth knows that Macduff's loyalties don't lie with him, and this makes him angry. He sends his murderers to kill Macduff's family. Macduff's wife is angry that her husband has left her and their children and tells her son that he is dead. A messenger shows up to try to warn her to take her children and leave, but she doesn't. She tells him:

"Whither should I fly? I have done no harm. But I remember now I am in this earthly world; where to do harm is often laudable, to do good sometime accounted dangerous folly: Why then, alas, do I put up that womanly defense, to say I have done no harm?"

Lady Macduff won't leave and in the end she and her children and servants are all killed. Macbeth thinks that, by the witches warning, Macduff was born by some supernatural force, however he was born by a cesarean section, so he was born by not supernatural deeds, but is the downfall of Macbeth after all.

Macbeth has two reasons for having Macduff's wife and children murdered. The first is to enact revenge against Macduff, who has fled to England to join Duncan's son Malcolm and assist in raising an army to attack Scotland and place Malcolm on the throne. The second reason is to make an example of Macduff and discourage other men from deserting him. In Act 4. Scene 1, Macbeth says:

The castle of Macduff I will surprise, / Seize upon Fife, give to th' edge o' th' sword / His wife, his babes, and all unfortunate souls / That trace him in his line.

This is typical behavior for a tyrant. It is the sort of terrorism Adolf Hitler employed during World War II when officers and enlisted men believed that the war was lost and were turning against him. Macbeth can no longer count on loyalty or patriotism and feels he must rule by fear.

Macbeth kills Duncan because of his "vaulting ambition" to become King of Scotland and his desire to please his domineering wife, who plots Duncan's murder and assists him in executing the bloody crime. Shortly after the witches greet Macbeth in act 1 by calling him Thane of Glamis, Thane of Cawdor, and future King of Scotland, Ross and Angus arrive and inform Macbeth that he has been given the title Thane of Cawdor, which coincides with the witches' prophecy. This news incites Macbeth's ambition, and he immediately entertains the idea of assassinating Duncan to become king. After Macbeth informs his wife of the favorable prophecies, she begins plotting Duncan's assassination.

Even though Macbeth desires to become king, his conscience initially prevents him from committing the crime and he decides to remain a loyal, trustworthy subject. During his soliloquy in Act 1, Scene 7, Macbeth admits that his "vaulting ambition" is his only motivating factor. Despite Macbeth's reluctance to obey his ambition, Lady Macbeth manipulates him by questioning his masculinity and assuring him of their success. After speaking with his cruel wife, Macbeth succumbs to his ambition and commits regicide by stabbing Duncan in his sleep. Following the assassination, Macbeth develops into a heartless, bloodthirsty tyrant, who continues to be manipulated by the witches and is eventually killed by Macduff during the final battle.

Macbeth first thinks about killing King Duncan after the the three witches prophesize that he will one day be king himself. After he hears this, he asks himself why he contemplates "that suggestion / Whose horrid image doth unfix my hair / And make my seated heart knock at my ribs, / Against the use of nature?" The "suggestion" he is tempted by is the idea that he should kill Duncan, which, as he says, is a sinful thought to have, and unnatural, or "Against the use of nature." He subsequently says in act 1, scene 3, that, "If chance will have me king, why chance may crown me, Without my stir." In other words, if it is his fate to be king, then he should simply let fate take its course and does not need to "stir," or help it along. This implies firstly that he was previously thinking about intervening and helping fate along by killing the king, and secondly that he has now decided that he will not kill the king, but let fate unwind as it will.

Macbeth does not find it easy to simply sit back and let fate take its course, without his help. He is, after all, a proud man of action, used to determining his own fate. In act 1, scene 5, he implores the heavens to "hide (its) fires" and "Let not light see (his) black and deep desires." Here then we can see that Macbeth still has strong and "dark desires" to kill the king, but we can also see that he tries to fight against those desires.

Macbeth's subsequent decision to succumb to his "dark desires" and kill the king can be largely credited to his wife. She decides that when King Duncan comes to their home the next day, he will not leave alive. However, in act 1, scene 7, Macbath acknowledges that he has only one reason to kill Duncan, and that is to satisfy his own ambitions. Indeed, he says that he has no motivation, or "spur / To prick the sides of (his) intent, but only / Vaulting ambition, which o'erleaps itself." Realizing this, he tells his wife that they "shall proceed no further in this business," meaning that they will not kill Duncan. Lady Macbeth nonetheless persists and accuses Macbeth of cowardice, asking him if he is going to "live a coward in (his) own esteem," and telling him to "screw (his) courage to the sticking-place." This proves to be all the persuasion Macbeth needs. He decides to kill Duncan after all and tells his wife that he is now "settled" upon that course.

In summary then, Macbeth's decision to kill King Duncan has two main causes. The first is his own "vaulting ambition," which first gives rise to his "dark desires." The second is his determination always to be the courageous man of action, which his wife exploits. His own ambition is the driving force behind the decision, but he needs his wife to give it a nudge in the right direction. In this way, his ambition is like a boulder teetering on the edge of a cliff, and his wife is the force which tips the boulder over the edge and starts the avalanche.

There are several reasons for his decision. Macbeth has been told by the witches that he will eventually become king. If Duncan is already king, how can Macbeth become king? Answer: kill Duncan. However, this is Macbeth's first mistake. The witches operate on the basis of fate--it is destined for Macbeth to become king. However, Macbeth takes matters into his own hands (rather than just letting events play out as they will), and that disrupts fate's system. (Or you can argue that it was fated that Macbeth would kill Duncan and that's what the witches were predicting when they said he's become king.)

Lady Macbeth also encourages Macbeth to kill Duncan. She is almost more status and power hungry than he is. She calls his manhood into questions, saying if he was a true man, he would do this. She uses all sorts of feminine manipulation to persuade him into killing Duncan--and Macbeth doesn't want to see like less of a man to his wife.

When the deed is actually to be committed, Lady Macbeth said she would have killed Duncan, but he looked too much like her father when he was asleep. Therefore, it leaves Macbeth to actually do the killing.

In act 3, scene 1 of William Shakespeare's Macbeth, Macbeth explains the reason why he intends to order the murder of Banquo, his onetime friend and comrade-in-arms, and Banquo's son, Fleance.

MACBETH. To be thus is nothing,But to be safely thus. (3.1.52–53)

Macbeth's priority up to this point in the play has been "To be thus" —that is, to become king. Now that Macbeth is King, his new priority is "to be safely thus"—to remain king.

Macbeth hasn't forgotten the prophecy that the witches made to Banquo at the same time that they prophesized that Macbeth "shalt be King hereafter!" (1.3.52).

THIRD WITCH. [to Banquo] Thou shalt get kings, though thou be none. (1.3.70)

This prophecy troubles Macbeth to such an extent that he mentions it twice to Banquo in that scene: once almost immediately after the prophecy is made, "Your children shall be kings" (1.3.89), and once again after Ross and Angus tell Macbeth that King Duncan made him Thane of Cawdor, fulfilling one of the prophecies that the witches made to Macbeth.

MACBETH. Do you not hope your children shall be kings,When those that gave the Thane of Cawdor to mePromised no less to them? (1.3.127–129)

Macbeth seems more concerned about Banquo's children being kings than with being king himself.

The prophecy isn't mentioned again until the beginning of act 3, scene 1, when Banquo himself raises the issue. Banquo thinks aloud about his suspicions that Macbeth murdered Duncan, "and I fear / Thou play'd most foully for't" (3.1.2–3), and he also notes that the witches said that no descendant of Macbeth will be king, "But that myself should be the root and father / Of many kings" (3.1.5–6).

With Banquo's short speech at the beginning of the act, Shakespeare tells the audience that Macbeth has two reasons to fear Banquo. The first reason is that Banquo suspects that Macbeth murdered Duncan, and the second is that Banquo's descendants, not his own, will succeed him as king. It turns out that Macbeth is much more concerned with Banquo's descendants than with Banquo's suspicions.

Macbeth enters the scene and invites Banquo to his coronation feast, then casually inquires, "Ride you this afternoon?" (3.1.21). A few lines later, Macbeth asks, "Is't far you ride?" (3.1.26).

Macbeth knows that Banquo suspects his involvement in Duncan's death, and he weakly tries to deflect that suspicion by mentioning to Banquo that Duncan's sons, Malcolm and Donalbain, have fled Scotland, "not confessing their cruel parricide" (3.1.34–35). Then Macbeth asks, "Goes Fleance with you?" (3.1.39).

The reason for these incidental questions soon becomes clear. Macbeth engages two murderers to kill Banquo, "and with him... Fleance his son," while they're riding outside the castle.

Until Macbeth can rid himself of Banquo and Fleance, Macbeth wears a "fruitless crown" and carries a "barren sceptre" (3.1.65–66) and will continue to fear the prophecy that Banquo's descendants will be kings.

Macbeth has heard the witches' prophecies, and since their prophecies about him have all come true, he fears that Banquo's prophecy will also come true. Therefore, Macbeth perceives Banquo and his offspring as threats to his retention of the crown.

Macbeth has ignored Banquo's warning:

...the instruments of darkness tell us truths,Win us with honest trifles, to betray'sIn deepest confidence (1.3.133-135) 

He also has trusted in little things that the witches have predicted, although they have hidden larger and more dangerous changes from him. Macbeth now believes that he can put confidence in the predictions of the witches. This fatal confidence in the preternatural takes Macbeth into a dimension of the imagination that leads down a bloody and destructive path. In his developing paranoia, Macbeth feels that he must eliminate anyone who prevents his kingship. 

                     ....There is none but heWhose being I do fear: and under himMy genius is rebuked. (3.1.57-59)   

Because Macbeth perceives Banquo as a threat to his position as king, he sends for his henchmen and gives them the order to kill Banquo and his son Fleance. And so "blood will have blood."

Macbeth has Banquo murdered because he knows that he is suspicious that Macbeth might have killed Duncan.  He also was told by the witches that Banqo’s sons would be king.  This is the reason why he kills Fleance as well.

Even though Macbeth kills Duncan and takes the throne, he does not feel peace.  He is worried that he will be suspected, and that Banquo knows too much.  Remember, Banquo was there when the witches made their prophecy about Macbeth being king, and they also made their prophecy about Banqo’s sons being king.

Our fears in Banquo

Stick deep, and in his royalty of nature

Reigns that which would be fear'd. (Act 3, Scene 1, enotes etext p. 42).

Macbeth considers Banquo a threat because the witches said his sons would be kings.  This annoys Macbeth, who says “they placed a fruitless crown” upon his head.

Macbeth makes sure the murders know Banquo is their enemy.

So is he mine, and in such bloody distance

That every minute of his being thrusts

Against my near'st of life: (Act 3, Scene 1, p. 44)

Macbeth cannot kill Banquo and Fleance himself, because they are friends and they have friends in common.  So he hires the murders to do it.  After all, to Macbeth“to be thus is nothing/But to be safely thus” (p. 42).  In other words, what is the point of being king unless he is safe from enemies and potential enemies?

Macbeth's killing of King Duncan's guards in act 2, scene 3 leads Macduff to believe that Macbeth is involved in Duncan's murder. In act 2, scene 4, Macduff makes his suspicions about Macbeth known to Ross. Macduff tells Ross that instead of going to Macbeth's coronation, he's going back to his own castle at Fife, no doubt to look after his own affairs and to prepare for whatever Macbeth might have planned for him.

In act 3, scene 4, Macbeth reveals that Macduff refused to attend Macbeth's coronation banquet—the banquet which the ghost of Banquo attends uninvited—which angers Macbeth. Macbeth also reveals that he has spies in the homes of the nobility in Scotland, including Macduff's home, so Macbeth is fairly certain that Macduff suspects him of Duncan's murder and that Macduff is likely plotting with other nobles against him.

When Macbeth recovers his senses after being unnerved at seeing the ghost of Banquo at the banquet, Macbeth tells Lady Macbeth that he's going to the witches to find out what they know about his future.

In act 4, scene 1, an apparition conjured up by the witches tells Macbeth to "beware Macduff; / Beware the Thane of Fife" (lines 79–80). Macbeth already has concerns about Macduff, but before Macbeth can question the apparition further about Macduff, it disappears. A second apparition appears and tells him "none of woman born / Shall harm Macbeth" (lines 90–91), which Macbeth interprets to mean that no one, including Macduff, can harm him or imperil his throne.

Nevertheless, Macbeth vows to kill Macduff to "make assurance double sure" (line 92), if for no other reason than Macbeth can sleep better at night—"and sleep in spite of thunder" (line 96)—knowing that Macduff is dead and that he's no further threat to him.

Just a little later in that scene, however, Lennox tells Macbeth that "Macduff is fled to England" (line 158), which further angers Macbeth because he missed the opportunity to kill Macduff in Scotland, and now Macduff is out of reach in England. Macbeth resolves not to hesitate when he decides something must be done but to do it immediately. This leads Macbeth to his fateful decision to attack Macduff's castle, to "give to the edge o' the sword / His wife, his babes, and all unfortunate souls / That trace him in his line" (lines 168–170), before any more time passes and "before this purpose cool" (line 171).

Macbeth immediately sends murderers to Macduff's castle to do what Macbeth said he intended to do, which was to kill Macduff's wife and all of his children.

Consistent with Macbeth's firmness of purpose and his intent to attack Macduff's castle without delay, Shakespeare wastes no time in presenting that horrific scene to the audience in act 4, scene 2.

Since killing King Duncan and assuming the throne, Macbeth has become increasingly consumed with power and arrogance. He is not the same honorable person he was when in the king’s service. Now, he is willing to do whatever it takes to protect his crown—even if that means more murders.

When Macbeth sends word that he wants to speak to Macduff, he is rebuffed “with an absolute ‘Sir, not I!’” It is clear to the king that he cannot rely on Macduff’s loyalty.

Macduff poses a threat to Macbeth because he is working with Duncan’s son to raise an army to defeat the king. It is known that Macduff has traveled to England to meet with Malcolm. Therefore, Macbeth begins preparations for war, knowing it is the only way to retain his throne.

His resolve strengthened by the Three Witches, who tell him he will not be defeated until the woods travel to the hill, Macbeth decides that Macduff is an enemy to be removed. While he is confident that he can defeat Macduff in battle, he still worries about losing the throne in the future to any heirs who might seek revenge. Therefore, Macbeth reasons, the best way to remove any threat is to remove everyone; he gives the order to kill Macduff’s “wife, his babes, and all unfortunate souls / That trace him in his line.”

Shortly after assassinating King Duncan, Macbeth successfully usurps the throne and holds his coronation at Scone, where he will officially be crowned King of Scotland. Macduff suspects that Macbeth played a role in Duncan's murder and expresses his contempt by refusing to attend Macbeth's coronation. Instead of traveling to Scone and demonstrating his loyalty to Macbeth, Macduff travels to Fife.

Macbeth is offended by Macduff's decision to go to Fife and begins viewing him as a political enemy. In act 4, scene 1, Macbeth visits the Three Witches to learn more about his future, and the first apparition warns him to beware of Macduff. The prophecy confirms Macbeth's view of Macduff and solidifies Macduff as a serious threat. Macbeth then receives two more prophecies that influence him to become an overconfident, fearless tyrant.

At this point in the play, Macbeth has developed into a bloodthirsty, callous ruler who is determined to vanquish his enemies. In act 4, scene 2, Macbeth hires assassins to murder Macduff and his entire family. Tragically, Macduff's wife, children, and servants are brutally slaughtered. However, Macduff survives; he is in England attempting to convince Malcolm to challenge Macbeth.

One reason Macbeth slaughters Macduff's family is to send a message to his political enemies. He is motivated to portray himself as an intimidating, powerful tyrant who will punish and destroy his enemies. Macbeth also desires to upset and emotionally ruin Macduff, who is his primary opponent. Although he views Macduff as a threat, Macbeth believes that Macduff is not capable of defeating him. By massacring Macduff's family, Macbeth is baiting him to come back to Scotland, where Macbeth plans to defeat Macduff in hand-to-hand combat.

Ironically, Macbeth is not aware that Macduff was "untimely ripped" from his mother's womb—meaning that Macduff is capable of killing him.

At this point in the play, Macbeth has lost all touch with moralistic values and rational thought.  He is so fearful of losing his crown that he will do anything to protect it.  When Macduff was absent from Macbeth’s inauguration, Macbeth grew suspicious of him and decided to make a strong point by murdering Macduff’s family.  The reason for the murder of his wife and children was to clear the bloodline.  In Macbeth’s mind he did not want to lose the crown to anyone in Macduff’s family and the massacre would also send a strong message not to oppose him.  There is also a possible bit of resentment toward Macduff because he was able to do one of the basic primal objectives in life, reproduce.

What mainly concerns Macbeth is that Macduff has deserted him and fled to England to join up with Duncan's elder son Malcolm, who is seeking help from the English king to invade Scotland and claim the throne which rightfully belongs to him. Unable to get at Macduff, Macbeth has his soldiers slaughter Macduff's wife and children. This is not necessarily for revenge but to teach all the other thanes that if they follow Macduff's example they will suffer similar consequences. Macduff has shown from the beginning that he does not consider Macbeth the rightful king and that he believes he was responsible for Duncan's murder. Macbeth wants people to think that Malcolm had his father murdered and then fled the country to avoid punishment for patricide; but if Macduff joins Malcolm, he is showing that he considers this an outrageous lie. Macduff was the one who discovered Duncan's body, and Macbeth was with him at the time. Macduff remembers Macbeth's guilty behavior and knows intuitively that Macbeth was the real murderer.

Macbeth orders the hit on MacDuff's family only after he visits the witches again.  The witches call upon their spirit "masters" at Macbeth's request for more information.  The first apparition, a floating head, warns Macbeth:

"Macbeth! Macbeth! Macbeth! Beware MacDuff

Beware the Thane of Fife. Dismiss me. Enough" (IV.i.79-80).

Macbeth's motivation to kill the MacDuff family comes from that warning from the apparition.  Then in the same scene, Macbeth hears from Lennox that MacDuff has fled to England.  MacDuff's leaving seems to reinforce Macbeth's fears from the witches' prediction--that MacDuff is a threat that must be dealt with swiftly.  He immediately resolves to strike out at MacDuff and orders the attack on the castle to "give to the edge o’ the sword/ His wife, his babes, and all unfortunate souls/ That trace him in his line" (IV.i.168-170).  Macbeth's hasty decision to murder MacDuff's family reveals his growing paranoia; he will take whatever action necessary to ensure the end of MacDuff's line.  It also reveals a remarkable lack of hesitation on Macbeth's part; the man who agonized over murdering Duncan now easily orders the murder of an entire family.  Blinded by his thirst for power, Macbeth's morals have completely slipped away.  

Before Macbeth enters King Duncan's chamber to assassinate him, Lady Macbeth drugs his chamberlains so that they will not interfere with the crime. Both Macbeth and his wife plan on blaming Duncan's murder on both of his chamberlains, who would be completely unaware of their actions when they awake from their drug-induced sleep. However, Macbeth ends up stabbing and killing both of the chamberlains after he assassinates King Duncan.

In act two, scene two, Macbeth informs his wife that one of Duncan's chamberlains yelled "Murder!" while he was in the king's chamber and the other servant cried out "God bless us!". One could assume Macbeth feared that Duncan's chamberlains would wake up and witness him in the king's chamber, which is why he murdered them. If the chamberlains would have awakened to find Macbeth in Duncan's chamber, they would have surely blame the murder on him and ruined the couple's plan to usurp the Scottish throne.

After killing King Duncan and his two chamberlains, Macbeth carries the bloody daggers out of the room. Macbeth then refuses to return to Duncan's chamber and is immediately overwhelmed with guilt. Lady Macbeth then takes matters into her own hands by placing the bloody daggers back into the room and smearing Duncan's blood all over the servants. When the Scottish lords arrive to discover that Duncan and his two chamberlains have been murdered, Macbeth confesses to killing the chamberlains in a fit of rage to conceal his role in the king's death.

Just a short time earlier, Macbeth killed the king while the king was actually asleep in the same room as his chamberlains, who were quite intoxicated (Lady Macbeth added liquor to their drinks so as to get them so drunk that they would pass out and have no memory of the night). However, Macbeth tells her that, while he was in the king's room, committing the murder

One cried "God bless us" and "Amen" the other,As they had seen me with these hangman's hands,List'ning their fear. (2.2.37-39)

In other words, then, the chamberlains were not completely passed out and seem to have had some awareness of Macbeth's actions in the room. Why else would they suddenly ask for God's blessing in the middle of the night? It seems to imply some fear on their part. Therefore, it is not terribly surprising that Macbeth would kill them because they likely witnessed at least a portion of the murder. Macbeth tells the others that he killed them out of "fury" and implies that a truly loyal servant of the king could hardly be "reason[able]" when faced with the king's murderers (2.3.124, 130). He says that his "violent love" for Duncan made it impossible for him to remain calm in the moment (2.3.129). Of course, this is a convenient excuse for him to have killed them! Lady Macbeth helps to draw attention away from Macbeth by pretending to faint. It is also of note that Macbeth does not consult his wife in this— they never discussed killing the chamberlains—and he already begins to act on his own.

In act 2, scene 3, Macbeth tells Macduff that he killed Duncan's bodyguards because he saw them covered in blood, realized that they had murdered Duncan, and could not stop himself. In other words, he says that he was so overcome with grief over Duncan's death that he felt it necessary to kill the guards as an act of revenge:

For ruin’s wasteful entrance; there, the murderers,

Steeped in the colors of their trade, their daggers

Unmannerly breeched with gore. Who could refrain,

That had a heart to love, and in that heart

Courage to make ’s love known?

Macbeth claims, therefore, that this crime was one of passion. It was borne out of love for Duncan and that anyone who feels love would have done the same.

However, this is not the real reason for killing the guards. As the reader knows, it is Macbeth who killed Duncan. Killing the guards is, therefore, a necessary step in covering up his crime. Macbeth could not risk having any witnesses to this murder because it would ruin his chances of becoming king.

After Macbeth becomes king, he grows increasingly paranoid. Consumed by the desire to maintain his position of power, he constantly looks for signs of disloyalty in his thanes and even hires assassins to murder his potential political enemies or those (like Banquo) whom he judges to be threats. During Macbeth's reign of terror, Macduff leaves Scotland and travels to England, where he petitions Malcolm (the son of the former king) to return home and challenge Macbeth for the throne.

Determined to secure his reign, Macbeth consults the three witches in act 4, scene 1. When Macbeth meets with the witches for the second time, he demands that they give him more information about his future, and the witches conjure several apparitions and offer Macbeth seemingly favorable prophecies.

The second of these apparitions tells Macbeth to "be bloody, bold, and resolute" because "none of woman born shall harm Macbeth." Macbeth interprets this prophecy to mean that no one on earth can defeat him, because everyone is naturally born of a woman. Convinced of his own invincibility, Macbeth leaves the witches feeling reassured.

In the final battle of act 5, scene 8, Macbeth comes face-to-face with Macduff, his great rival. Macbeth warns his enemy that he has "a charmèd life," which must not yield "to one of woman born." However, Macbeth does not realize that Macduff had a cesarean birth and was thus "from his mother’s womb / Untimely ripped." This was not considered a natural birth at the time, and so since Macduff was not naturally born of a woman, the protection of the prophecy doesn't apply to him. When Macduff announces this, Macbeth realizes that the witches have misled him, but he still refuses to yield. The two men duel, and Macduff proceeds to kill Macbeth, later walking onto the stage holding the deceased tyrant's head.

Macbeth is put on the path to murder when he encounters the witches, who greet him with three titles: "thane of Glamis," "thane of Cawdor" and "king hereafter." At this point in the play, he has not yet received news that he's been granted the thaneship of Cawdor. This would come soon after, when Ross and Angus arrive after the witches depart.

Prophecy is a critical theme in Macbeth. It is prophecy that sets in motion Macbeth's usurpation of Duncan, and Macbeth's belief in prophecy also drives the death of Banquo. Still later in the play, Macbeth is given more prophesies, this time regarding his demise.

Ultimately, though, it is Macbeth's own ambition and his desire for kingship (a desire which the Lady Macbeth shares) which is responsible for the murder of Duncan. For example, in Act 1 Scene 4, Macbeth refers to his "black and deep desires." Later, in Act 1 Scene 7, he refers to his "vaulting ambition, which o'erleaps itself."

In addition, we should also add the influence of Lady Macbeth, who is fully committed to killing Duncan and rebukes her husband when he expresses second thoughts about the murder.

Macbeth kills Duncan because the witches first give him the idea, his wife gives him the plan, and both (idea and plan) reinforce his blind ambition to be king.

Macbeth has doubts about killing Duncan: he is his kinsman (relative); he has honored Macbeth of late (given him the title Thane of Cawdor for valor in battle); plus, Macbeth is his host and subject.  Murder obviously violates the Thane-King bond ("comitatus").  Not to mention that Duncan is meek and virtuous.

But, against all this Macbeth says (Act I, scene vii):

I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'er-leaps itself, / And falls on the other.

Also, Lady Macbeth challenges Macbeth's manhood and stirs up his bloody courage ("screw your courage to the sticking place").  She calls him a coward if he doesn't go through with her plan.  She says he is "too full the milk of human kindness" and admits that she would "dash out the brains" of her own child "had I sworn as you / Have done to this."  Such is her resolve.

After this, Macbeth agrees to kill Duncan, saying:

I am settled, and bend up Each corporal agent to this terrible feat.  / Away, and mock the time with fairest show: / False face must hide the false heart doth know.

In other words, Macbeth knows he must go against Time and put on an act to cover up his heinous intentions.

Fear. In his soliloquy of Act III of "Macbeth,"  Macbeth admits this to himself:

To be thus [king] is nothing, but [unless] to be safely thus--/Our fears in Banquo stick deep,/And in his royalty of nature reigns that/Which would be feared....(III,i,48-51)

Since hearing the predictions of the weird sisters, Macbeth has been motivated by amition and fear.  After all, it is Lady Macbeth who promises to give him the courage to enact his first murder:

This night's great business into my dispatch;/Which shall to all our nights and days to come/Give solely sovereign sway and masterdom. (I,v,67-69)

Although Macbeth has been described as brave on the battlefied, his "bravery" is ferocious, he overkills in his fear that anyone retaliate against him.  After hearing that he will not be killed by a man "born of a woman,"  Macbeth, ridden with both fear and guilt, becomes a bit paranoic.  As Frost wrote in his poem "The Road Not Taken," "Yet knowing how way leads to way," Macbeth's murderous path to ambition has no end but his own.

Macbeth has been told my the witches that he will not be killed by a man born of a woman, but to beware of Macduff. While Macbeth chooses to use the witches predictions as an indication that he will reign as king for a long time, he is still disturbed by the warning against Macduff and wants to ensure that he is completely safe. Therefore, he decides to kill Macduff and Macduff's family. Ironically, Macbeth's failed attempt to kill Macduff but successful murder of Macduff's family is what finally spurns Macduff to kill Macbeth himself.

In Macbeth, Macbeth acquires enemies as the play moves forward. The more he lies and kills, the more enemies he creates for himself. In Act IV Scene 1, Macbeth has had enough. He is so tired of living in constant fear that one of his enemies will overtake him and kill him. Therefore, in Act IV Scene 1 Macbeth goes to the witches and demands that they tell him his future. He wants to know what is going to happen to him. In this prophecy, the witches tell Macbeth to beware Macduff. Macbeth is so paranoid at this point in the play that he is willing to do anything to keep his kingship safe. Therefore, after the witches give this prophecy, Macbeth says,

From this momentThe very firstlings of my heart shall beThe firstlings of my hand. And even now,To crown my thoughts with acts, be it thought and done:The castle of Macduff I will surprise,Seize upon Fife, give to th' edge o' th' swordHis wife, his babes, and all unfortunate soulsThat trace him in his line. (Act IV Scene 1)

In this quotation, Macbeth is saying that he is going to act on what he heard from the witches right away. Since the witches said to beware Macduff, he decides to raid Macduff's castle and kill him and his family. He decides this huge decision in a matter of seconds because of his paranoia and the witches' prophecy.

Macbeth believes that Macduff is suspicious of him and he is trying to stop him from intervening.

Macduff is a noble who is at Macbeth’s castle on the night he kills Duncan.  Macbeth is worried about the men who were there, including Banquo and Macduff.  He is concerned that they will be suspicious that Duncan’s murder was actually committed by the Macbeths.  When Banquo speaks up, Macduff agrees. 

Fears and scruples shake us:In the great hand of God I stand; and thenceAgainst the undivulged pretence I fightOf treasonous malice.

MACDUFF

And so do I.

ALL

So all.

MACBETH

Let's briefly put on manly readiness,And meet i' the hall together. (Act 2, Scene 3) 

Macbeth therefore sends murderers after Banquo, his son Fleance, and the Macduff family.  Macduff happens to not be there. He is with Malcolm, the king’s son, who fled to England to gather an army to oppose Macbeth.  Therefore, the murderers kill Macduff’s wife and children.

Macbeth has no conscience.  He thinks nothing of killing men who think they are his friends, and their children (or wives).  He works with villains like the murderers because he does not want to get his hands dirty himself.  All he cares about is maintaining his throne now that he has it.

Macduff is with Malcolm when he learns what happened to his family.  He is horrified that he lost all of his family at one time.

All my pretty ones?Did you say all? O hell-kite! All?What, all my pretty chickens and their damAt one fell swoop? (Act 4, Scene 3)

Malcolm tells him to “Dispute it like a man,” which basically means he has to fight Macbeth, and in that way he will accomplish their goal and avenge his family.  Macduff actually manages to hold it together pretty well. The witches have told Macbeth that he needs to beware Macduff, but they also said no man born of woman could hurt him.

When Macduff meets Macbeth in battle, he tells him that he was “from his mother's womb/Untimely ripp'd” (Act 5, Scene 8).  This subdues Macbeth, who gives up hope from then on.  Macduff beheads him, and Malcolm takes his spot as king.

By this point, Macbeth is unable to be rational in his judgments or to be able to tell good from evil. He sees Macduff as a threat to his future, so he feels taking quick action against him and his family will insure that any blood line that might threaten his future will be stopped. He doesn't stop to think about what he is doing. He murders in order to stay powerful and in control. The senseless murders of Lady Macduff and her son show how much Macbeth has degenerated into a cold-blooded killer. These killings reinforce Macbeth's character flaws which lead to his tragic end.