To understand human nature, is to understand the natural man. The natural man is selfish and cares little about others unless it directly affects them. Hopefully religion of some sort helps people to overcome the natural man. It takes work to become peaceful and loving the neighbor and helping instead of trying to harm or better ones self at the cost of another. Like it was stated before, violence comes naturally. Lack of understanding or compassion for others opinions can also lead to frustration which will lead to violence. Some people do not know how to cope, or ignore other people without getting under their skin, and again some of those same people do not know how to let things go and they let it build up until the breaking point when they have convinced themselves that violence is the only option.
Violence can also be a tool for the righteous, when the time comes to cut fence and sort bastards out, you want good men very skilled in violence.