Perceived age and art

Frankly, fiction is fiction. I may not personally approve of or have any interest in it, but as long as real people/animals/whatever aren't harmed, I don't really care and it's not my right to care.

If someone uses AI art-- or any fictional media, for that matter-- to keep from hurting and exploiting real, living things, then let them.

Next step.

Let's assume, that a person is indeed a nazi, sadist, pedophile, racist, necrophile, potential killer, obsessed with violence, or homosexual (oh... that's not a crime anymore...), or all of those at the same time, but haven't committed any of those in real life.

What would you do or suggest doing with/to such a person?

Nothing.

If they want to be treated for it (because things like pedophilia and necrophilia are fetishes, not sexualities, and those can be cured with the right kind of therapy) then let them seek treatment without getting stigma thrown at them, but if they're entirely non-offending, have not hurt or exploited anyone, and do not seem imminently likely to change that, then nothing necessarily needs to be done.