Why are people so cruel?

There's this boy at my school who has, down syndrome or autism. Anyway he really gets into school and likes to talk and when he talks he stutters, and everyone laughs at him and makes fun of him..Sadly I've seen teachers even laugh at him. The kid is really nice and very smart. I seem to be like the only one who doesn't make fun of him and is nice towards him. You think people would be really nice, it's the total opposite. It makes me feel so sad the way people treat him. Why are people so darn cruel?