This is a perverted view of history. In fact, in small hunter-gatherer tribes it was easier to keep track of each other and hold each other accountable, so crossing someone would be risky. Today, with large societies that are increasingly anonymous, one can more easily get away with screwing people over--you can always move someplace else. So I don't think being nasty is particularly natural for humans, it just comes out in the right conditions where people are likely to get away with it.
Outcasts are vulnerable, yes, but that's why you try to prevent becoming one. I have seen Dogville, but I don't see why this fictional anecdote would refute my thesis which is based on what I believe is the consensus in modern anthropology. I didn't imply that 'nothing nasty would happen to anyone', that's a straw man. I merely pointed out a contrast between small close-knit groups vs. large anonymous crowds.
I think it does take more. It takes provocation or discontent. The point about being able to get away with it was that it can make a lynch mob more vicious than they would have been individually without the cover of anonymity. Whether it really is natural or not is not a well defined question, but I believe that people left to their own devices, when they're not hungry or otherwise deprived, wouldn't display nasty behavior.
