Sometimes I wondered if it was just that Kal-El had a constant personality. A sort of approach where being Superman (personality not powers) was a given with or without the Kents. Give him to Stalin to raise and he still is altruistic, things don't work out for him there but he definitely isn't a Soviet Ultraman or Ubermensch. The only hitch is that is undermined by stuff like Injustice where we see a Superman who does go dark.
It stood out most in the Elseworld where he is raised by Darkseid of all beings and still winds up noble. I mean if Darkseid can't turn Superman evil ... ?