Two things about this.
Firstly, superman helps people cause it's the right thing to do. We understood that much from his rhetoric about the school bus, he was raised(by someone) to ask the simple question, 'what was I suppose to do, let them die?' Good parents, unbroken household, and an access to things like Plato, I would understand that being enough to explain why he feels the drive to helps folks when he can. However this does little to explain what he 'feels' is right when his first premise(helping folks) comes into conflict with a new premise, that of taking a single life to do so. Does a good man take a singular life to save the universe? What's the right thing to do here. Neither Jon Kent(literally) nor Plato could give you that...
I'm sure a good man like Stark or even Parker could tell him.
Feeling that killing is wrong when you are out there fighting for the universe takes a specific explanation beyond, "I don't need to kill to know..".
Having the kill shot on the detonator man in a superbowl goes beyond, "I don't need to kill to know.." It takes someone that knows the answer and who know that answer better than someone that knows what it is to kill? Someone that doesn't?
Look at all the colourful marvel heroes that kill for example. Not killing isn't inherent in doing the right thing, it takes a characterization catalyst if a film is actually in the game of dramatizing imo.
t:
