The remark “If I looked like you I’d kill myself” is very mean and shouldn’t be used by anyone. I imagine it could be said by a bully that is trying to make somebody feel bad about the way they look. If we were to look at the phrase from a more light hearted point of view, I imagine maybe some friends were going to a costume party and one of them had a very ugly costume, making the others laugh and say they would never want to actually look like that.
