[...snipped] How do you feel morally about revenge? Is it ever justified? Is it a matter of degrees and proportion, or is it just never okay?
How do you feel practically about revenge? Is it ever necessary? Can it ever be healing?
How do you feel experientially about revenge? Do you like it? If so, how do you go about getting it? Are you good at it?
If you still have reason to feel threatened by the other person, then definitely take appropriate measures to secure your safety. That's self-defense, not revenge.
But if you're secure from any further harm from that person and you are simply contemplating "getting even," then here is what the self-help books prescribe:
Revenge is usually a bad idea. Revenge can draw you into a cycle of retributions back and forth, basically keeping you in a relationship with someone who does you harm.
On the other hand, emotions don't recognize time. An old injury done to us by another person can upset us 20 years later as much as it did at the time. So it may not be enough simply to secure one's safety. There may be a need to do something to get past the old anger. After all, you don't want to be dwelling forever on old injuries.
The book "Boundaries: When To Say Yes, How To Say No To Take Control of Your Life" by Dr. Henry Cloud & Dr. John Townsend (published 1992) talks about the need to practice forgiveness. That is, you "let it go" and free your head of the old injuries. Of course, forgiveness doesn't mean you let your guard down around that person. Forgiving isn't the same as reconciling. You can forgive someone and still keep your distance from them and distrust them, since they haven't repented or made amends. Forgiveness only requires one person; reconciliation requires two. You should forgive and move on in order to free your head, regardless of how you feel about the individual. (p. 256-7)
At the same time, the authors acknowledge that forgiveness can be tough. For example, the book emphasizes that it may be necessary to go through a grieving process in order to forgive properly. "To forgive means we will never get from that person what was owed us." (p. 268)
It also means giving up blame. When you have difficulty giving up blame, "it says that you can never be okay until someone else changes. This is the essence of powerless blame. It may make you morally superior to that person (in your own thinking, never in reality), but it will never fix the problem."
Okay, fine. What if you're currently in a relationship with someone and you're having trouble forgiving and forgetting petty injuries and insults from the kinds of petty clashes and head-butting two people routinely experience when together?
Basically the idea is to 1) Establish secure personal boundaries to head off repetitions of preventable injuries; and 2) Cleanse your head of resentment over old boundary violations and rip-offs by your partner in the past by practicing forgiveness. Do all this, and you can attain some kind of zen state of equanimity and detachment. From there, you can evaluate clear-headedly how your boundaries are working out and where to go next: Stay in a relationship with that person or dump them.
IOW, get rid of the old anger, set up boundaries to prevent new anger, and you'll be in a better state to evaluate and see whether the relationship is worth saving.
Disclaimer:
Again, I'm just quoting the self-help books here. Obviously this kind of one-size-fits-all advice won't work for everyone.