his own wife has insulted his manhood. This isn't well-received in our own modern society, and it certainly wasn't well-received by men who fought battles of valor to the death in hand-to-hand combat. She provokes him in his hesitation, noting:

When you durst do it, then you were a man;
And to be more than what you were, you would
Be so much more the man. (1.8.54–56)

Macbeth feels that he has to defend his manly sense of honor and strength, not cringing in what his wife labels as cowardice. Lady Macbeth's stab at her husband's pride is deep and achieves its intended purpose.

Yet Macbeth does, in fact, hesitate. He doesn't want to kill Duncan because he knows that by doing so, he jeopardizes his afterlife. He also aptly reasons that:

But in these cases
We still have judgment here, that we but teach
Bloody instructions, which, being taught, return
To plague th’ inventor. (1.7.7–10)

Violence leads to further violence, and Macbeth realizes this. If achieves his ambitious goal through murder, he realizes that his kingdom is thus prone to further acts of violence, even if the people never know who committed the act.

King Duncan trusts Macbeth, so murdering him is doubly vicious. And King Duncan has proven to be a good leader:

Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off. (1.7.16–20)

The reasons for not killing Duncan clearly outweigh Macbeth's own personal quest for glory; however, his wife's final push thrusts him forward into violent actions.

Reasons to kill Duncan:

• Macbeth has proved himself to be a brave warrior on the field of battle. The time is ripe for him to take the throne. It's a case of now or never.
• The witches' prophecy was clear: Macbeth would become Thane of Glamis, Thane of Cawdor and ultimately king. The first two prophecies have come true, so it is reasonable to expect the third one to materialize too. But it's important not to leave too much to fate; sometimes you need to give it a helping hand. So Duncan must die.
• If you're going to be king, you need to be able to prove that you can act ruthlessly sometimes. It comes with the job. And what more ruthless act can there be than regicide?

Reasons not to kill Duncan:

• Macbeth, like all other Scottish nobles, swore an oath of loyalty to his king. By killing Duncan he will violate that solemn oath. That sets a dangerous precedent, because if nobles start showing disloyalty towards their king then it will undermine the stability of the whole kingdom, whoever's on the throne.
• The witches' prophecy concerning Macbeth becoming Thane of Glamis and Thane of Cawdor came true without his needing to do anything. Fate just took its natural course. So instead of killing Duncan, Macbeth should simply bide his time and have faith in what fate will bring.
• Killing Duncan will undermine the stability of Macbeth's throne. He'll quickly earn the reputation of a tyrant, uniting nobles against his regime. And the only way to consolidate his power will be to resort to even greater repression and bloodshed. In turn, this will simply make his opponents all the more determined to overthrow him.

After receiving the prophecy from the Three Witches in Act 1, Scene 3, Macbeth immediately begins thinking about assassinating King Duncan once he learns that he has been named Thane of Cawdor. Macbeth's foremost motivation for killing Duncan is to become king of Scotland. Macbeth craves the authority and absolute power a king possesses, which motivates him to commit regicide. Macbeth is also motivated to fulfill his prophecy, enhance his social status, please his wife, and leave an impressive legacy.

Despite the numerous reasons that support Macbeth's desire to kill King Duncan, there are also many opposing reasons for Macbeth to not follow through with the murder. If Macbeth commits regicide, he will more than likely live a life full of guilt, grief, and regret, given the fact that Duncan is his cousin and murder is against natural law. The social order of Scotland would also be upset, and chaos would likely result from Macbeth's unjust reign. Furthermore, the Three Witches' prophecies might not be correct; they could lead Macbeth down a dangerous path. Macbeth would still be forced to kill Malcolm, and Duncan's assassination would only lead to further bloodshed. Macbeth's soul would also be doomed if he were to kill the king, and Macbeth would only create more enemies.

Yes, Macbeth has the prophecies.  He seems to think that he should be king because he has been told that he is.  He was basically told three things:  He was Thane of Glamis, he would be Thane of Cawdor, and he would be king.  Since the first was true and the second came true, he figures the third should be too.

Some reasons MacBeth would want to commit regicide and kill King Duncan:

1. To gain power over all of Scotland and advance his station in life.

2. To achieve the crown.

3. To fulfill the prophecy.

4. To look like a "man" in his wife's eyes.

Some reasons NOT to kill Duncan include

1. Murder is against natural law.

2. Killing a King will disrupt the Elizabethan cosmology and chaos would soon result.

3. Disorder, chaos, and unnatural acts will be loosed on the world.

4. The Weird sisters may not be a reliable source of information or prophecy. Why should he trust them?

5. Guilt and a stained conscience will inevitably result in psychological ailments.