What does Macbeth name as the sole reason for wishing to kill duncan?

In Act I, Scene 7, Macbeth is apparently trying to talk himself out of going ahead with the murder of King Duncan. In his soliloquy he names all the possible reasons why he should not do it. He foresees serious consequences. The killing would not be the be-all and the end-all. He would be setting a bad example which others could follow. He has an obligation as Duncan's host and kinsman to protect him, not...

In Act I, Scene 7, Macbeth is apparently trying to talk himself out of going ahead with the murder of King Duncan. In his soliloquy he names all the possible reasons why he should not do it. He foresees serious consequences. The killing would not be the be-all and the end-all. He would be setting a bad example which others could follow. He has an obligation as Duncan's host and kinsman to protect him, not kill him. Duncan has been an admirable king. His murder would arouse widespread outrage. Macbeth concludes this soliloquy by saying:

I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other—

He has no legitimate excuse for murdering King Duncan. His sole reason for wishing to kill him is his ambition. By calling it "vaulting ambition," Macbeth seems to be using the image of a rider who is showing off by jumping onto his horse's back but jumps too high in his swaggering and falls on the other side.

When Lady Macbeth appears, her husband tries to explain why he does not want to go through with the planned assassination. He gives another reason to add to all the others he has thought of for not killing Duncan. He tells his wife:

We will proceed no further in this business:
He hath honor'd 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.

He is saying, in effect, that their ambitions should be at least temporarily satisfied with all the honors he has received. But her ambition is greater than his. She overwhelms his rational arguments with an emotional personal attack. She reminds him that they have Duncan under their roof and at their mercy--something which might never happen again.