I am not sure that Macbeth really ever does have an 'a-ha' moment where he decides that he should be king because he would make a better king than Duncan. Really, his driving force of motivations to kill Duncan is his own ambition, and one of the reasons why he does feel so incredibly guilty is because Macbeth does recognize that Duncan is not only a really good king, but also just a good person. In fact, Macbeth is so troubled by this realization that he worries:
"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" (I.vii.16-20).
Ultimately, Macbeth's decision to murder Duncan, even though the true king is such a good ruler is the moment when Macbeth destroys the natural order of things and makes matters worse. Clearly, Macbeth recognizes that Duncan is the better choice; however, his greed and ambition convince him to usurp the throne, even though Macbeth recognizes the moral wrongness of his actions.