I once tripped a kid on purpose
Why would you deliberately do such a thing?
Because I was no longer interested in being his friend, he had been mocking me too much.
You shouldn't hurt someone physically in retaliation for anything. You should have communicated with this person to find out why they were mocking you, getting in your side of the situation with them.
I think what you're saying is pretty stupid. Hurting others is a great idea to get back at them.
I don't think I expressed myself properly there.  What I meant was, that even though this incident has already occurred, you should have tried to talk to this kid, and find out why he was mocking you.  Hurting him would just have made him dislike you more, but talking could potentially have solved the problem.