Actually, not killing IS a Kryptonian trait. It's why once civilization started to form on Krypton, capital punishment was the first thing to go. And it's why Krypton was so advanced-because not only was Krypton scientifically advanced, it was also socially advanced.
"There was a right and a wrong in the Universe and that distinction was not very difficult to make."-Miracle Monday.
Superman's code against killing is one of the bedrocks on which Kryptonian philosophy is built. And it goes hand-in-hand with Superman's main belief: "Do good to others and every man can be a superman."-that is "the way" that Jor-El speaks of Superman showing humanity, and that is how humanity can, in Superman's view, transcend itself. Transcendence through altruism. Superman's philosophy is a mixture of Kryptonian principles and the Kent's ethics. And it is the Kent's ethics, not just "midwestern farm ethics", like any farmer could have raised him so well. Ma and Pa Kent were to morality and ethics what Jor-El and Lara were to science.