I think stopping the analysis at the point of recognizing that the threat is ambiguous misses the bigger picture. It has to do with cognitive dissonance and avoiding guilt. There is zero guilt in wanting to avoid a serial killer on the loose. However wanting to avoid our socially awkward cousin may give us feelings of guilt if we try to rationalize it. Is there really anything wrong with him? Can we precisely say what it is that makes him unpleasant? The feeling of being creeped out by him allows us to justify avoiding associating with him without over-analyzing why we don't like him, which might make us feel guilty if we can't really put our finger on why. Creepiness allows us to bypass feelings of guilt over our own potentially irrational dislike of things. Is it irrational to have such a hatred of all spiders? Perhaps, but I don't have to confront it, they're just creepy and that's all there is to it.