“But Brutus says he was ambitious; and Brutus is an honourable man.”
Julius Caesar III.2
Better slow than never: here’s another probe into the world of Brutus:
Why does Brutus go along with the assassination plot (and in fact become its de facto leader) even though Cassius’ motives are so obviously personal, and despite his own apprehension?
Another excellent question. I think it boils down to the fact that Brutus has set for himself an impossibly high standard of personal conduct: his vaunted “honour”. He is in a constant state of emotional suppression and denial (even so far as denying himself any opportunity to grieve the horrific suicide of his own wife) because he believes that, by setting an example for those around him, he will eventually save humanity (or at least Rome) from those needs and desires which lead to conflict and bad judgement. He seeks complete mastery of ‘Ego’ over ‘Id’. (The irony, of course, is that the human failing which Brutus seems to find most egregious is ‘personal ambition’, and what could possibly be more ambitious than to seek to transcend one’s own flawed humanity?)
So, why does he kill Caesar? Because, in Caesar’s acquiescence to ambition (and flattery, and superstition, and anger, and envy, and suspicion, and all of those negative emotions that come with being human), Brutus is forced to acknowledge his own potential fallibility, and he is afraid that what he sees will eventually be too horrifyingly tempting to overcome through sheer willpower. He sees the picture of what he might become, and has to destroy it.