Excellent, intelligent response.

My feeling is that when perpetrated in the heat of the moment (e.g., you just came home and discovered someone in the midst of doing evil to your family members), then revenge is nature's / God's way of empowering a person (or even a dog) to stop on-going assault and to prevent future crimes by the evil-doer.

It's a natural response and one with a positive purpose, which is why juries often see it as a mitigating factor.

Two minutes later, it's another story.