Why are we so cruel?

My son told me Wed after school that a boy in his school had died. The boy killed himself because he was being bullied in school. My son told me about two incidents: He said that he was knocked down in the hallway and kicked by other kids in school. A girl told him that he should go away and just kill himself. There was more, I'm sure. My son didn't know him well. He is very upset about what the other kids did. I asked him if the boy told anyone about what was going on. He said no, but that the teachers knew about it and some of them tried to stop it.