I think that's totally at odds with the OP's argument. Under the OP's view, atheists aren't scary because they might be violent -- most people aren't psychopat

I think that's totally at odds with the OP's argument. Under the OP's view, atheists aren't scary because they might be violent -- most people aren't psychopaths, that's not what social signaling is for -- but because they've chosen to chuck out a large chunk of the signaling machinery that the "middle middle class and below" have habitually relied upon. In other words, if you've sworn off the dominant canon of ethical and social judgements, your position now suddenly requires just-in-time evaluation by the other, and that is threatening to people who don't trust themselves to reliably pull it off.