As a child I was teased constantly because I was skinny. (Even as an adult I sometimes got rude comments about that, mostly from people who were overweight.)
The other kids in grade school didn't mention my asthmatic wheezing, which could usually be heard all over the classroom, but that's probably why they were uncomfortable around me. I thought of myself as the class victim and it never occurred to me that I, too, was a bully.
No, I never physically hurt anyone or threatened to do so, but I often "defended" myself by making sarcastic comments or said things laden with sarcasm because I thought they were funny. Looking back, I realize I must often have hurt the feelings of other kids.
Although I can scarcely remember the physical threats made against me, most of which never actually happened, I can still remember some of the nasty comments my classmates made. And I'm afraid some of them might remember similar things I said to them.
A while ago I mentioned a website that gives wonderful advice about how to deal with bullies. On rare occasions in my childhood and as an adult I'd used the tactics suggested there and can vouch for the fact that they do work. I strongly recommend http://www.bullies2buddies.com.
As an adult I try hard never to say things that might hurt someone else's feelings because I don't want to be a bully. And if someone says things like that to me, I know how to react.