SUPERMAN discovers that his father Jor-El invented a machine that could siphon the life energy from distant planets, killing all their inhabitants while increasing the health and vitality of Kryptonians. It was used several times before it malfunctioned, starting the chain reaction that ultimately destroyed Krypton.
Also, in their earlier days, Jonathan and Martha Kent were members of the Kansas KKK, and were involved in blowing up an African-American church, killing several people. Racist violence was simply part of their culture in those days....
BATMAN, investigating a decades-old religious cult of serial killers, discovers that his parents, Thomas and Martha Wayne, were its founders. The person who killed them before his eyes in Crime Alley was a survivor of their murderous practices. I think we can all agree that, if Bruce has to overcome this horrendous background, it makes it even more impressive that he becomes such a great hero. He decides not to bring the remaining cult members to justice because, firstly, he sort of thinks of them as family, and, secondly, who are we to judge somebody else's religion?
I mean, why not?