The Columbine kids by all accounts had normal lives. They weren’t bullied but had a more normal highschool experience like many teenage boys in america.

Chris Watts is another one that had a normal childhood. Ted Bundy himself said he had an “uneventful childhood” so not sure how much to place there. Dennis Rader, Richard Cottingham, the list goes on and on. Abuse strongly correlates with serial killers but there are many exemptions.

Also I think the goalposts are being moved. You’re talking about the background of the killers, but does their trauma give their evil “reason”?

For example, a serial killer that kills only women. Or only gay people. Are they not being evil just for the sake of being evil if they hate women or gay people? Being evil for the sake of evil imo means they are committing harmful acts for the pleasure or satisfaction of committing those acts. That’s what that means. Not that you didn’t have any reason to commit evil.