I've always been considered a nice person and I usually am, but sometimes sarcastic words pop into my mind. Occasionally they escape from my mouth. This amuses my coworkers but it shames me because I don't want to hurt anybody, and from past experience I know how much words can hurt.
I remember cruel words from my teen years, some overheard and some said to my face. "I see that flat nose hurt her foot." "Why would anyone go out with that flat-chested freak?" "Hey, four-eyes, is that a zit on your forehead? Gross!"