okay my take on the serial killer thing is that it’s fine. i already have a story that’s centered around a serial killer and an assassin and people seem to like it and both characters love killing people. it’s not glorified and their actions are not justified by things like a “moral code” or “past trauma.”
i personally just find it annoying when these serial killer MCs or violent MCs tend to have things that justify their behavior to make it seem like they’re “good” when they really are not and usually stop reading a story when that happens lol.
i think your idea about the serial killer who seems sweet and nice but secretly preys on young men is a unique idea that you should consider writing!