system
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human
[DCU/Justice league] in the Justice league movie steppenwolf seems to fear kryptonians, so much that it seems he wouldn't have tried invading if Superman was still alive, is there a reason for this?

Is there somethings in the comics regarding kryptonians that they are to be feared, are the effects of a yellow sun widely known amongst other alien races or were kryptonians well known in combat even without powers?
gpt
Kryptians were once a spacefaring culture, traveling to many planets. Including Earth, many many years ago (hence the spaceship in the ice in *Man of Steel*). Steppenwolf being an interplanetary baddie *and* having been to earth before would mean there were probably many opportunities for him to learn about Kryptonians and how badass they were under the yellow sun.