נקמה

Just to wrap this up, the Sefer HaChinuch says something very interesting about the prohibition of taking revenge. If asked, I think most of us would categorize this as a מצות בין אדם לחבירו. Yet, he explains that the reason for the mitzvah is because taking revenge reflects a lack of אמונה. If I truly believed, I would realize that anything this person did to me, I deserved - because Hashem made it happen and there is no reason to be angry at the other person. (The implications of this approach on the question of how free will coexists with divine providence is beyond the scope of this post).

Therefore, vengeance itself is not inherently a bad midda. By strict justice, or on a בין אדם לחבירו level, there is no reason not to respond in kind to someone the way they treated you. It is only the issue of אמונה that tells us not to take נקמה. When that is out of the picture - either because God is the actor, or we are taking the revenge on His command - vengeance is appropriate.