It should be preactive but proactive is too well established. Yeah, that's why I used pro-.
Pretaliation is better.

What I was thinking of is along the lines of you seeing a person whom for some reason you don't like, and you go do something bad to them even though he was just minding his own business and you had never seen him before. As in, "All you X's are such-and-such, therefore you deserve what I'm about to give you".

There are several words for criminal charges for doing this, but none that I knew of for this specifically-motivated act.