I agree that we shouldn't ban characters on principle, based solely on their canon or what they do there. If they can be functional in the game, then really, by all means, let them in. But I can see where the anon above is coming from, as I think the problem they're having with Caster is that his entire deal is that he murders/tortures/arguably rapes children. There basically are no other traits, outside of his fixation on Saber, that we're given in canon.

Hannibal Lecter isn't exactly comparable here, because he has other traits - he's enough of a psychological clusterfuck that things can be done with him in-game outside of the cannibalism aspect. This is more comparable to apping, say, the villain from Enzai, who does have traits that make him a valid character and able to be apped, but at the end of the day he's a rapist from a yaoi game. That is why he exists, first and foremost - to rape and torture the character. A lot of people would find his existence in the game itself to be triggering, or at least enough to make them very, very wary and made of side-eye.

I'm not saying you don't have a valid point, because banning characters based on canon would result in a lot of lines being drawn that you understandably do not want to draw. I'm just saying that I think this is where they're coming from - it's not "ban all negative characters!", it's "this character exists only to carry out actions that serve as triggerbait, it bothers me that they're here."