I can't decide between the following three:

1. Kicked my elderly grandmother off the top of a bunk bed when I was in the 8th grade. And then laughed about it later.

2. Systematically destroyed a girl's belief that all people are inherently good while also destroying her innocence. Then I made fun of her dead grandfather and laughed as she cried.

3. Carried on a charade for over half a year with a girl who was in love with me. At the culmination of the entire thing, when she was admitting just how much she was dedicated to me, I told her that she was not attractive, fat, and that I did not like her. Then I told her to go away.

I don't really consider them to be entirely horrible myself, although some people might.