containing or implying a slight or showing prejudice
"Activist judge" as a disparagement is invidious in that it is intended to show that the judge has given to someone else that which belongs to the hearer -- it casts envy upon the hearer.
a longing for something better than the present situation
When an audience hears a judge accused of activism, the audience is discontent because of the dissonance between the ideas of authority of the judicial system and the idea that militant activism can overthrow the judicial process.
"Activist judge" conjures the idea that the judge is unfair in his process and the parties in the suit should not accept the judgment, thus animosity continues.