To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
Macbeth knows that he has no reason to kill Duncan except for his own ambition: Duncan is a good king - "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" (note the euphemism 'taking-off" which shows Macbeth is hiding behind language because he knows what's he's doing is wrong".