What motivates Macbeth to start killing?

I don't believe that there is one definitive answer to this question. Different readers/audience members will possibly feel differently about exactly what most motivates Macbeth to commit the first murder. Often, Macbeth's personal ambition to have more fame, fortune, and power is blamed. He is definitely excited by the prediction that names him future king. He doesn't believe it is possible until Ross and Angus give him the news about being a new thane. Macbeth then realizes that he just might be king, and he's excited about it.

Glamis, and thane of Cawdor!
The greatest is behind.

He's so excited about the possibility that he almost immediately begins to consider killing Duncan as a way to get to the throne faster. Macbeth wants to be king, and he's motivated enough by that prospect to consider murder.
Unfortunately, I do not believe that Macbeth's ambition and desire for the throne are enough to motivate him, and Lady Macbeth knows this about him as well.


Yet do I fear thy nature;
It is too full o' th' milk of human kindness
To catch the nearest way: thou wouldst be great,
Art not without ambition, but without
The illness should attend it.



In fact, by act 1, scene 7, Macbeth has convinced himself that he is not going to go through with the murder, and he tells this to Lady Macbeth.


We will proceed no further in this business.
He hath honored me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.



Lady Macbeth is so not excited to hear that. She wants to be the wife of a king. She wants queenly powers and all of the perks that go with it. She proceeds to insult Macbeth and calls into question his manhood.



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. Nor time nor place
Did then adhere, and yet you would make both.
They have made themselves, and that their fitness now
Does unmake you.


By the end of her chat, Macbeth is once again motivated to kill Duncan; therefore, I do think a lot of Macbeth's motivation comes from his wife. It could be fear of her. It could be that her speech convinced him of the plan being a good plan. It could be that he wants to prove his manhood once again to his wife. It could even be the adage "happy wife = happy life." Regardless, without Lady Macbeth's "pep talk," I don't think Macbeth would have been motivated enough to go through with the murder.