Having been intrigued and enticed by what the three weird sisters told him, Macbeth has written to his wife of his being made Thane of Cawdor as well as their prediction that he will be king. But, while he is excited at the prospect of being king, Macbeth is also disturbed by the workings of the supernatural world in which "nothing is /But what is not" (1.3.) and these "new honors" disturb him. Later, in Scene 7 in his soliloquy, Macbeth deliberates about killing Duncan:
If it were done when 'tis done, then 'twere well
It were done quickly....
but this blow
Might be the be-all and end-all--here,
But, here, upon this bank and shoal of time,
We'd jump the life to come.... (1.7.1-6)
Macbeth is concerned about several things:
- Regicide is a serious crime because murdering the ruler of the country is a grievous offense.
- Regicide also upsets the Chain of Being that the Elizabethans believed in, a chain in which the king was God's deputy on earth. So, the unlawful death of a king causes a disruption in the world of nature, and can bring plagues, famine, and unnatural activity among animals.
- It was considered important to know one's place in the Chain of Being and not try to rise above it because of ambition, which was unholy.
- King Duncan is a kinsman of Macbeth's and so Macbeth is reluctant to kill him.
- Macbeth is host to King Duncan and should be preventing any harm to the king, not harming the king himself.
- King Duncan is virtuous and heaven will surely punish him who kills this king.
For all these reasons, Macbeth hesitates about murdering Duncan; finally, he concludes that it is only his "vaulting ambition" that motivates him.
Macbeth gives a number of reasons why he should not kill Duncan. He had many more reasons for not killing King Duncan than for carrying out the killing.
For example, he owed the King respect as as cousin, host, and subject. Killing the King disrespected Duncan's position as benefactor, cousin, guest, and king.
Additionally, the King was beloved and respected by the people of Scotland, and they were likely to be outraged by his murder.
Finally, if people got the idea that you could become king by killing the king, wouldn't they do the same to Macbeth if he became king in this way? (And of course this is exactly what happened.)
In addition he says to his wife that he should enjoy his new titles while they are still new.
There was only one reason that Macbeth could offer himself in favor of the murder. That sole reason was his ambition, and Macbeth didn't think much of it, calling it "vaulting ambition which o'erleaps itself".