Godwin's law.
Yeah, hate to give him any credit but Hitler really is #1 on the scoreboard as far as that's concerned. The sheer insanity of it all, to think that so many people could get roped into it. It's a cultural paradigm shift of the phenomenal variety.
But what do I know, perhaps what happened in those times wasn't so far-fetched all things considered.
Least common red flags, that's an interesting question and I'm not sure. I've known people that I'm certain have the capacity for great destruction, but I'm not sure any of them are murderous. After all, what makes me uncomfortable is totally okay to some people.
It's a sort of double edged question. The more I think about what uncommon queues people may give off socially could be, the more I have to acquiesce to my undoubtedly flawed judgement and blatant hypocrisy.
I can say that I haven't been murdered yet, having made friends from many walks of life. I like to think my intuition with people is pretty keen, so here's a few of the things I don't like. Not necessarily in relation to murder.
People that have no respect for physical boundaries I almost never hang out with. I'm pretty sure most guys hate being manhandled, but it's never surprising when one of the people that are all happy go lucky with their physical contact are willing to ball up their fists on you.
People that justify primary dimensions of prejudice have proven they're capable of killing for the most irrational reasons. This includes anyone from any demographic, age or race.
Narcissists are a definite red flag for me too, even if they are actually really sexually attractive. I get flexing in the mirror after you worked out or whatever, but people that post videos or photos talking about how fabulous they are. They often expect the world to fall at their feet, which sucks because it often does. But if they don't get what they want, oh boy.
Nihilism, is another one. I actually dislike my own deep sense of nihilism and find most optimists endearing. Nihilism is often in the belief that nothing matters. Think about it, nothing matters. If this is taken to the extreme, things get fucking ugly.
A little bit of nihilism can be healthy though, like grating a little fresh ginger into a whole pot of soup.