I tormented my littel siblings when I was a teen and I feel bad for that now and have apologized since for it. Seemed like nothing at the time.
When I was little I also twirled my kitty by his tail and didn't realize how cruel that was at the time (I was 6yo). Kitty is long since gone but I still feel bad I did that to such a loyal and good pet.
When I was in high school one of my friends didn't like another friend of mine. One day one friend decided to pour fruit punch on my other friends head. I knew what was going on the whole time but didn't stop it. That's one mean thing that comes to mind. I'm sure I have more I just can't think of them right now.