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Someone so hated that no one will testify if they are murdered is definitely an Asshole Victim, so that stands regardless.
The situation overall has elements of Screw the Rules, I'm Doing What's Right!, with "Rules" in this case being the law and the "Right" being determined by the community.
In general, "character dies or has a bad thing happen to them; the rest of the community reacts positively" is And There Was Much Rejoicing. It goes both ways, so good guys can be happy with a bad guy getting their comeuppance, and bad guys can be happy with a good guy forcibly getting off their case.
If a crime ends up going in front of a jury and the jury pulls a Jury Nullification (the defendant definitely performed the deed exactly as described, but the jury feels it was justified or the law itself is dumb), probably Common Nonsense Jury.

Do we have a trope for "A person commits a crime, but because the community thinks it is justified/acceptable, nobody will testify against them"? This can be both positive (protagonist murders evil character, victim is so hated by community that nobody testifies) or negative (evil character murders innocent, society is so bigoted that nobody testifies". It's something described in the book "Omerta" by the concept of the same name, but it is seen in several media.
There is overlap with Asshole Victim for sure, but it's not exactly that, I think.
Edited by arcanephoenix