As someone has already said, I'm sure we've all participated in bullying to some degree, depending on how we define the concept. While I never sought someone out and deliberately tried to upset them, I certainly did go along with the school mob mentality thing when it came to picking on the "misfit" kid. Never to his face, but I went along with the bitching and insults behind his back. I regret it now, but I was a young teenager and didn't think it through. He's actually at the same university as me now, though I've only run into him once. He's not the sort of person I'd choose to be friends with still, but we had a decent enough conversation and he doesn't seem to bear a grudge against me (though like I said, I wasn't one of the ones giving him shit to his face).
I did once punch the school bully in the face when I was about 15 though, and I'm still proud of it, whether or not that might count as "bullying." The guy was an ass, seriously. He picked on EVERYONE except his little group of friends. Not just "nerds" or "easy targets" or whatever, literally everyone in our year outside the "popular" crowd (who couldn't stand him either, but he thought he was one of them). I didn't get the worst of it, just name calling and attempts to make me feel stupid, and though I couldn't stand the bastard what he did and said to me didn't bother me all that much. But one day I was just sick of him seriously upsetting my friends and classmates, and told him to shut up. He turned around and started bad-mouthing me, and I punched him in the face. In a crowded corridor. Hilariously, I completely got away with it because the rest of the students who saw/heard about it were very much on my side, and there was no way the "hard man" was going to tell a teacher that he'd been hit by a girl. He never bothered me again, either. He even tried to befriend me. Happy days.