Jury nullification occurs when a jury returns a Not Guilty verdict even though jurors believe beyond reasonable doubt that the defendant has broken the law. Because the Not Guilty verdict cannot be overturned, and because the jurors cannot be punished for their verdict, the law is said to be nullified in that particular case.
What he did was right. Yes, much like how Hitler’s death would’ve been sad for Eva Braun had she still been alive, Brian Thompson’s death is sad for those who survive him, but the world is a brighter place for it.