I’ve often thought that this would be a great theme for a novel, to-wit: at what point does a good man decide that he must rebel against the existing structure of society, and do so violently? This is not as easy as it might seem. Evil men have always justified their deeds as righteous efforts at protecting themselves from the depredations of a society gone bad.
Thoreau and Martin Luther King wrote extensively about (and lived out) that very issue.