Zod and co. were actually arrested in Donner cut, so Superman didn't kill them...
Originally Posted by Poni_Boy
the thing is, the Man of Tomorrow angle doesn't hold up in this film. From what I remember in the comics, Krypton was a peaceful planet where killing was forbidden. That's part of the whole Man of Tomorrow mythology. Earth is supposed to use Superman's example of no death penalty tactics to eventually become more like Krypton. But by removing it from the film's mythology, it's a mute point.
If you look beyond that, Krypton is what Earth could be like in the future except for becoming so full of pride, they'd ignore the warnings of one their greatest scientists. Superman as the Man of Tomorrow shows this example and Earth becomes what you see in Legion times: like Krypton, but without the pride.