There is one thing that makes people feel enough rage to commit violence, and that is a feeling of powerlessness. If people feel that they have no control over their destiny and environment, if they feel that they cannot act effectively, then they can reach a point where they believe that nothing short of violence can change their situation.
Acting effectively requires you to influence other people and to control your environment. To influence other people, they must respect you and be willing to listen to what you have to say. To control your environment, you must understand it, have the skills to affect it, and be permitted to act on it.
It should be clear that these conditions are not met very often in our society. Many people in our society are alienated from one another and have few opportunities to exert any real influence on one another. Many poor and uneducated people do not have any control over their environment whatsoever.
However, powerlessness is not the only ingredient in violence. The real question is not why people are violent, but why so many men are violent. Although women are just as capable of violence as men, crime statistics show that it is not women who are turning our urban environments into war zones.
Both men and women must abide by certain expectations. Even though people have few instincts and all of our adult behavior is learned, we labor under the misconception that men and women are biologically destined to behave completely differently. Women are supposed to be yielding, they are not expected to forcefully express their own wants and needs. Men are supposed to be dominant and commanding, and are regarded as weak if they express any tendencies to yield or to behave in a "feminine" way.
As psychologists have discovered, however, the most mentally healthy people express emotional and behavioral characteristics traditionally assigned to both sexes. The fully functioning human can be either forceful or gentle, commanding or submissive, strong or yielding, as the situation requires. Unfortunately, the acceptable range of emotions for men is rather narrow, and what happens is that men must express all of their emotional energy through the few emotions available to them. This leads to rather exaggerated expressions of strength and virility.
Now, couple this self-image men have of strength and domination with the feelings of powerlessness rife in our society, and you have a recipe for disaster. Men must express their exaggerated sense of dominance, but they are rendered impotent by their inability to act with any effectiveness. To these men, violence seems to be the only way to affect their environment.
This will continue to be a problem until men are raised differently.