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I know; I know; I know. This is exactly what makes this topic so frustratingly difficult to explain, and so convenient to ignore.
The thing I am trying to say is that if a real monster would come to this community, sufficiently intelligent and saying the right keywords, we would spend all our energy inventing alternative explanations. That although in far mode we admit that the prior probability of a monster is nonzero (I think the base rate is somewhere around 1-4%), in near mode we would always treat it like zero, and any evidence would be explained away. We would congratulate ourselves for being nice, but in reality we are just scared to risk being wrong when we don't have convincingly sounding verbal arguments on our side. (See Geek Social Fallacy #1, but instead of "unpleasant" imagine "hurting people, but only as much as is safe in given situation".) The only way to notice the existence of the monster is probably if the monster decides to bite you personally in the foot. Then you will realize with horror that now all other people are going to invent alternative explanations why that probably didn't happen, because they don't want to risk being wrong in a way that would feel morally wrong to them.
I don't have a good solution here. I am not saying that vigilantism is a good solution, because the only thing the monster needs to draw attention away is to accuse someone else of being a monster, and it is quite likely that the monster will sound more convincing. (Reversed stupidity is not intelligence.) Actually, I believe this happens rather frequently. Whenever there is some kind of a "league against monsters", it is probably a safe bet that there is a monster somewhere at the top. (I am sure there is a TV Tropes page or two about this.)
So, we have a real danger here, but we have no good solution for it. Humans typically cope with such situations by pretending that the danger doesn't exist. I wish we had a better solution.
I can believe that 1% - 4% of people have little or no empathy and possibly some malice in addition. However, I expect that the vast majority of them don't have the intelligence/social skills/energy to become the sort of highly destructive person you describe below.