I think the reason is that we believe vigilantism encourages more vigilantism, and that can get really ugly, really fast, and I think we're not wrong.

I remember a group of vigilantes in Seattle receiving some positive press a while ago. 

In general, I suspect you’re right though. 

Why do I think vigilantes aren't popular? While Batman is indeed popular, I find that most people only enjoy it as a fantasy. When faced with the reality of an "unauthorized person" using some sort of force to enact justice (especially on a non-comic-book scale) most people are shocked. 

Note: this argument of mine is based solely on the feels. I have no evidence to back it up.

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Batman’s one of the most popular characters of all time. What made you think people don’t like vigilantes?