The Damn Nation Art Trope

The dark Phoenix saga in the X-Men where apparently I was reading up on this and the writers were actually kind of debating internally whether Jean Grey is possessed by an evil force or whether she just turns evil. Sometimes there's this idea that it comes from outside and she's taken over and other times it's just something dark within her that eventually manifests itself right? You see this idea a lot in cosmic horror stories where you have a character like the guy in HP Lovecraft's short story Shadow over Innsmouth who comes to this town and they're full of Cthulhu's spawn.