People who call others ugly are just ugly on the inside, back in middle school I was picked on a lot, I was on the chubby side and had bad acne. It caused me to become pretty self conscience about the way I look. I think being called ugly hurts so much because first of all, it seems as if someone is saying that they are in some way better than you. Also because they are placing judgment on you.
Being called ugly does hurt, but to be honest, negativity is nothing more than a sign of insecurity, so try not to let those silly haters get you down.