Some, but not all, are born into violent behavior. They see it everyday and it becomes "norm". People who are exposed to violent behavior definately have a higher chance of becoming violent themselves.
Some people have a chemical inbalance which causes them to act violently, while others are introverts. They never fully developed a personality making it difficult for them to interact with others in a positive way.
Some may have a mental problem.
Some acts of violence can be purely out of jealousy. This is sort of like a "crime of passion". These people are driven by passion and under normal circumstances would not have acted out.
There can be numerous reasons as to why some people act out in a violent way.