//“Part of the blame must lie with...” Funny, in most cases right wingers think the blame lies with the person who commits the crime//
The "part of" bit is an attempt to explain the violence - as in motives, causes, provocations. An intelligent approach some might think. Example: "Why did the Palestinian attack and kill the Jewish grandmother with a knife?". Answer: "It was the desperate reaction of a person who has suffered injustice and oppression at the hands of the Israelis. What he did was wrong, but , while not approving, we understand why such people, might be driven to commit evil acts".
I think the poster and his spiritual siblings get all that, because I've seen just such rationalisations used, if not as defences, at least as pleas in mitigation of violence.
The cause of vigilantism is the perception that serious crime is going unpunished. What's wrong with vigilantism is not that it seeks redress for a crime, but that when it fails to find the actual perpetrators it will seek out and punish (as in this case) innocent, but symbolic surrogates for them.