I agree with the above statements, however I will add another contributing factor. How often was in when you were younger that you were bullied? Did you stand up to him/her? I was bullied when I was in school and when it got to a certain point, it came to blows. The bullying stopped. Children these days are taught to talk about their feelings. If you don't like what somebody is doing to you, talk to them about it. More often than not, this mentality doesn't work and it teaches our children that it's unacceptable to stand up for yourself. Eventually, that uncontrolled bullying turns in emotional rage. Then you have extreme violence.
I'm not saying physical altercations are the right way to handle everything, but there are times when it is called for. But, there are times when there are no alternatives.