Where to start...
There was one time I beat up 5 boys that were 1-2 years younger than me (I was roughly 10-11 years old then) once, at the same time (they were wimps). They were annoying me every day, and I had enough. They never bothered me again, and they still don't look me in the eyes. That's 5-6 years ago now.
Something I feel more bad about though, is one time I beat up one of my best friends. He was really annoying me, and I snapped. I threw him around a little, before I threw him on a low fence so he hit it with his stomach, and then I took me elbow and hit his back with it as hard as I could. We didn't speak for some time... Luckily we're good friends now.
One thing I feel worse about was one time when I was 10. We had an assignment at school to write a card to someone. For the last few days a friend of mine had been really annoying, so I wrote a card saying this in a.. hostile manner. It was percieved as a death threat, and he was really scared that I'd hurt him (he's really skinny and non-muscular, while I'm somewhat muscular, and at that time was quite strong compared to other people I know). Of course I didn't mean it, I was just annoyed. I sincerely apologised though, and we're really good friends today (we're going to an In Flames and Killswitch Engage concert in December ^^). I am to this day his only friend who has not done anything with the purpose of hurting him (hitting, kicking him and things like that). Every other person he knows has at least once hit him with the intent to make him hurt (preferably cry I imagine, as he used to be prone to that).