I can’t remember anyone justifying out of personal dislike or popularity. Their justification is that this person’s actions are mass social murder, suggesting that such anti-social people deserve to be killed if the legal system refuses to punish and deter them. The fact that people generally hate a mass killer is incidental, it’s not the reason they deserve a punishment.

There are good arguments against vigilantism in general, and while I don’t fully agree in this specific case, I respect them as valid reasons. But to say this assassin is being given a free pass just because people don’t like them is absurd.