There's a certain point where it goes from teasing to something else entirely

There was a kid I went to high school with who was in a friend group I was also in. There was a point where we started calling him names just as a joke to mess with him. It got to a point where he became emotionally distant, but we kept doing it anyway just bc we didn't think much of it. His personality completely changed. He went from being a happy person to a shell of a person. We were his only friends at the time. He was still distant when we weren't around. He's (apparently) still like this also

I still feel sick to my stomach about that situation to this day, but tl;dr there's a line at which teasing goes from just teasing to something horrible