[Rodrigo90]

Not Superman related, but I felt one of Batman's explanations for not killing suits Superman best.

When young Bruce Wayne asked Thomas Wayne why he saved the life of a criminal, when he could have turned him away. Thomas said he believed that all life was sacred, no matter who's it was. It was his duty, not just as a doctor, but as a good man, to preserve life.

So Bruce believed in his father's philosophy, to preserve life and not end it, even if it was the life of an evil person, and the belief of the good within himself, that stops him from crossing that line into evil...hence why he doesn't believe in killing or leaving evil men to die. Because it's his duty as a good person, to do the right thing. That's why he can't be swayed by his personal feelings of anger, its his philosophy on life itself.