I can remember only one occasion when I was intentionally mean to someone--I got very very angry, and then coldly decided to say the meanest thing I could possibly say. I wanted them to feel worse than I felt when they said something very mean to me. The difference was that their unkindness was unintentional. Astonishingly ignorant, but nevertheless unintentional.
I was pretty effective--the person cried all day, or so I heard. I'm glad that now I feel so much safer in the world than I did then, and now I have an internal place-to-stand from which I can notice my reaction and be accepting and fascinated about it.
It's totally fascinating to me that the great majority of other people feeling pain in response to my words and actions has been besides, rather than because of, my intention--that is, that generally I hadn't set out wanting them to feel pain.