I've been vengeful myself but not acting out on it turned out to be the best thing. People that hurt others on purpose or are hurtful by nature eventually wind up digging a lonely, miserable grave for themselves. I have perfect proof of it in the person I felt vengeful towards. I never lifted a finger to do anything against this person and he has paid a thousand times over for what he did to me and others.

The best way to set yourself free from this is to forgive. Yes, forgive. It stops the control that others have over you. When you get yourself to the point that what they did to you is no longer important, then you are truly free. Chances are, they are still living their hurtful lives and will come back to them, like someone else said, threefold, if not more. What you need to concentrate on is healing for yourself.

Rise above the situation and set yourself free from it. If not, you'll end up like them.