how old were you when this happened, what happened to trigger it? what did you do to them?
for me, it was when i was 18 (11 years ago) - a guy I was in halls of residence with had been bullying me, spreading malicious gossip about me, breaking my stuff, etc etc. one night i came home a bit drunk and found him lying on the floor, having broken my chair and wrecked my room. something in me just snapped and i stamped on his chest quite hard... it was one of those moments where you instantly regret your actions so i'm quite grateful that (a) i didn't actually cause him too much damage and (b) he didn't fight back. it's the one and only time in my adult life i've hit someone.