In Macbeth, Macbeth is lauded as a hero for defeating the enemy and for showing his undying loyalty to his king - Duncan - who calls him, "valiant... worthy gentleman" (I.ii.24). Duncan is so impressed that he sends his men to award the title of Thane of Cawdor to "noble Macbeth" (69). The audience has already been exposed to the witches' appearance, and the "foul and fair a day" (I.iii.38) during which the audience is introduced to Macbeth foreshadows later events when Macbeth takes matters into his own hands.
After the witches have told Macbeth what future awaits him and that he shall be king, Macbeth is conflicted as he is both excited and uneasy at the prospect. The thought of being king plays on his mind to the point that it gives him what he calls "earnest of success" (132). He is unsure whether, having received the first title (Thane of Cawdor) through honorable means, he will be awarded the kingship "without my stir" (143), or must find his own way to ensure that the title of king follows.
Even before Macbeth tells Lady Macbeth, he is again considering his position and his options, wondering about Malcolm, Duncan's heir, and his potential to prevent the witches' prophesies from coming true. He reflects that his "black and deep desires" (50) encourage him to find a way to overcome this obstacle. However, Macbeth will recognize his "vaulting ambition" (I.vii.27) as being key to his murderous thoughts and, despite Lady Macbeth's encouragement, he decides to "proceed no further" (30).
Lady Macbeth, however, has a profound effect on him and he cannot bear the thought that she may consider him to be less of a man. Together they devise a plan but it is now Lady Macbeth who is driving it. Before murdering Duncan, Macbeth's new status as a villain preoccupies his thoughts, rendering him delusional. He imagines the daggers but fights the fear and commits Duncan to "heaven or to hell" II.i.63). Having killed Duncan, Macbeth is so tortured that he imagines he "hath murdered sleep" (II.ii.43). In his confusion, he still carries the daggers which he used to kill Duncan. At first the audience may think he is remorseful, unable to return the daggers himself in his altered state and wishing to wake Duncan "with thy knocking" (73). However, despite his continued disorientation, he is already becoming a calculated murderer as, shortly thereafter, he commences his arrangements to have Banquo murdered.
Macbeth transforms into a down and out killer with no feelings of fear after he visits the witches again. He has witnessed Banquo's ghost but, after the witches fuel his ambition to be king, he believes he is invincible. Lady Macbeth is no longer necessary to cement his future and even her death will have no effect on his resolve.
In constructing his play, Shakespeare must have realized that it wouldn't do to have Macbeth become a good king after murdering Duncan. There may have been a few people who suspected him of the crime, but for a short time he was in a secure position as the legally elected king. If he had been a beneficent ruler--as no doubt he initially intended to be--he might have gone on ruling for the rest of his life. And if he and his wife could manage to produce a son, that boy would succeed Macbeth as the lawful monarch. The English king would not want to interfere in Scottish politics merely to help Malcolm claim the throne which was rightfully his. For all the English king knows, Malcolm might have been guilty of bribing the grooms to murder his father. Shakespeare knew that Macbeth had to become a villain and a tyrant in order to meet his downfall. This would give Malcolm and Macduff the justification to overthrow him and give the English king the motivation to supply them with the means to do it. They needed an army. Macbeth should have tried to be an ideal monarch after he usurped the Scottish throne. What turned him around was that he couldn't stand the thought of Banquo benefiting from his assassination and usurpation. He had disgraced himself in his own eyes and sold his soul to the devil for the benefit of Banquo and all of Banquo's descendants.
He chid the sisters,
When first they put the name of King upon me,
And bade them speak to him; then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding. If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!
The other thanes were apparently content to have Macbeth as king and to accept the fabrication that Malcolm and Donalbain had been responsible for their father's murder. The thanes, naturally, were concerned about their own titles and properties, and they needed a king to secure them. But when Macbeth makes a deplorable scene at the inauguration banquet, it becomes blatantly obvious to everyone that he was responsible for Banquo's murder and therefore was probably responsible for Duncan's as well.
Thou canst not say I did it: never shake
Thy gory locks at me.
Prithee, see there! Behold! Look! Lo! How say
Why, what care I? If thou canst nod, speak too.
If charnel houses and our graves must send
Those that we bury back, our monuments
Shall be the maws of kites.
After this scene Macbeth loses the trust of all the thanes. If Macbeth can murder Banquo and probably murder Duncan, then who is safe from this madman? Macbeth is then forced to rule by terror, and this is the turning point, the beginning of the end for him. He makes matters worse by having Macduff's whole family slaughtered by his soldiers, intending to make Macduff a lesson to anyone else who might be thinking of deserting him. In the end he has no one he can trust. The witches encourage him to be "bloody, bold and resolute," but, after all, they have been the agents of his ruin ever since he first met them on the blasted heath.
I have lived long enough. My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honor, breath,
Which the poor heart would fain deny and dare not.
We feel some sympathy for Macbeth after he murders Duncan. After all, he is not behaving like a villain at that point. He is tormented by pity, guilt, shame, fear, remorse, nightmares and hallucinations. But after he has Banquo murdered and tries to have Banquo's son Fleance murdered along with him, we start to lose our sympathy, just as the thanes will lose theirs and all turn against him.