Admit it, guys: we can all be mean and cruel and heartless when we're children. So share your stories about how awful you were.
I remember in eighth grade, there was this girl in my class I went to a different school with when I was younger and she was mean to me then and mean to me at the time. (We made up by the end of high school.) One girl who had this class with us always saw us bickering and she swooped to my side to defend me from her. Being socially awkward at the time and not knowing how to say "Thanks, but I've got this," I said "No offense, but mind your own business."

That started a WAR for the rest of the year. We were back and forth at each others' throats, and at one point I remember calling her a stupid bimbo Barbie. Anywaaay, by the end of the year we kind of forgot about it, and I never ran into her in high school. We only talked briefly maybe once after that year. I saw her at a friend's wedding a few months ago and thought about going up to apologize, but I was like naaaaah because she probably didn't remember.
In ninth grade, there was this girl who initially seemed like one of the sweetest people you could meet. Everybody liked her. Some boys in my class who didn't like me influenced her into not liking me either, so we eventually got into a feud. She was mostly the one who was awful to me, but one day in a fit of rage after she told me I had no friends, I told her, "You're never going to amount to anything in life." She cried in the bathroom during lunch that day. I initially felt bad when some girls told me that, but then I realized she deserved it.

Our ninth grade class was out of control with me getting into fights with people, so eventually our teacher staged a mass "therapy" session towards the end of the year, so it was all pretty much resolved and I barely saw the girl after that year.
And one last story: there was this really nerdy boy in my homeroom class in ninth grade. I didn't think I was overtly mean to him, but I did join in some of the bullying in an attempt to fit in with the "cool" kids. The one really awful thing I did though - and this was BAD - was steal his homework and put my name on it, while he had to take a 0. (I wasn't targeting him specifically, I just forgot to do my homework and didn't want a bad grade!) I think he found out later.

I found out just how much he hated me at the end of the year when he spread a hit list around the school and I was #16 on it.

He got suspended for the last two weeks of the year and I never saw him again after that.