As a member of society I wouldn't want people I don't know/trust/didn't accept as authority out there doing what they please to appease their own morals. I don't really care that they believe they're just - what I care is how it affects me and the society I live in. And vigilantism inherently has a huge margin for making things worse. Because vigilantes are human, they're not necessarily stable mentally and even if they are they don't necessarily stay stable and sane; there's no guarantee that their morals won't change with time, or that their power won't get to their head, that they won't harm innocents in their attempts to remain anonymous, etc. etc. It's a very slippery slope, and once you step on it it's guaranteed not to end well.

It's not a question of appealing to the masses or not. If we get down to it, it's about all the things that can potentially go wrong, and ordinary people not living in fear or at least uneasiness because there's a murderer on the loose - a well-intentioned murderer, maybe, but a murderer all the same. (And if he starts being less well-intentioned and more concerned with his own purposes, it's even worse.)

I'm not saying that vigilantism is necessarily wrong by default, but vigilantes resorting to the heavy end of violence and murder? I can't condone that. There are just way too many things that can go wrong.