Interesting questions.
When is it ok? According to my Christian parents, it is not ok, but I do sometimes and I think they did too.
What does it do for me? It makes me feel that I'm right and that feels good. It makes me feel better than the other person.
When do I seek revenge? I'm passive. I guess I would rather soak in my feelings of superiority.
How do I retaliate? See above.