I’m no stranger to dark subject matter, and I’m willing to write a story as dark as a typical South Park episode (or at least as dark as Disney’s Hunchback), but there are some themes I wouldn’t even go into no matter how dark my stories are.
That being said, the one type of characters I would never write are pedophiles. It’s disturbing, gross, and too dark for my stories. Besides, if I make an interesting villain who is savagely cruel, making him a pedophile would make people lose interest into the character. In all my past, present, and future works, there would be no pedophiles ever.
I also would avoid writing villains from actual evil groups like Nazis or Al-Qaeda. As evil as they are, they’re evil for a reason. Making them the bad guy in a story that doesn’t relate to them nullify this point. But if I do want to write characters based on them, I can come up with a fictional group that’s just as bad as they are, but also portray them realistically to show why the groups they’re based on is evil.