What have you ever done that you really felt guilty about?

Roasting.

At my high school, if you can roast, you're a god. At first, I could hardly insult someone.
Then, I started trying to come up with roasts in my freetime, until eventually I was practicing roast battles with friends, and then started doing the real thing. It was a roast battle with some random kids at my school, but eventually it got out of hand.

Step on my foot?
I roast you.

Cut me in a line?
Roasted.

Until one day, I roasted this girl.I roasted her on her hair, clothes, etc.

The next day, she came back with different hair and a whole new style of clothes. I felt so bad that I completely changed someone by insulting them.