The kid with the pimples picks on the fat kid, who picks on the gay kid, who picks on the slow kid, etc. To make matters worse, when we hear someone put down someone else, most people laugh along because they are so fearful of being the next person to be picked on. The perpetrator hopes the nasty comments will drag the other child below them in the social pecking order. In time, kids realize that this technique does not work. Nor does it make them like themselves any better.
Negativity leads to more negativity, and the beat goes on.
Some people have the courage to speak up when there is hateful talk. At a minimum, silence, instead of laughter, sends the message that you are not amused or entertained. Millions of dollars are being spent trying to determine why some children snap and randomly lash out at their classmates. It has never been a mystery to me. We have all been victims of man's inhumanity to man.