Why are people so aggressive and violent towards each other? I think, that it is because violent scenes are often presented in mass media, television and in computer games. There is blood and dead bodies in many films and games. Many heroes in the movies have to kill others in order to survive. In the end, such hero wins all the fights, destroys everyone and everything around. Several movies and computer games are based on such scripts. In most computer games, the player controls this hero, identifies with him. Such person learns that killing is not a bad thing and that it can make you famous. I think that if young children look at these movies and play such games, they consider killing a normal thing to do. They want to do it as well. Some times ago I read in the newspaper about two ten-years old boys, who were playing "Duke Nuken" (the name of the computer game) and one of them killed the other. Afterwards he said that he thought he would help him "get to the other part" doing this. Some kids may do such things, because they want to find acceptation in the group, impress their mates. They need to show others that they are cool and that they can do anything they want.
In gangs and similar groups, being violent is the way to career. It helps them survive. It is like in the jungle. Only the strongest survive. It happens that people, who come from poor families and don't know what to do in their future, join such groups. They quit school and decide to become criminals. One can notice, that each year there are more and more such people.