I think that violent behavior is a combination of nature and nurture. I think that people with the violent gene, if one exists, are not inherently violent. These people might have the violent tendency but I do not believe that the tendency will surface without the proper, or improper, nurturing to make that violent tendency raise its ugly head.
I do not believe that there really are any violent people that were born that way. As I said before, I believe that many people have some kind of gene that causes violence which makes people meet Violent. But, I think that the gene will not be triggered until it meets certain conditions.
Scientists really do not know much about the human psyche. Psychology and other sciences of the mind are only in their infancy and we have only just touched the surface of understanding how the human mind actually works. Sure, many scientists want us to believe that they actually do know what they are doing when they are only actually making guesses and estimates. If they truly understood the human mind, we would see an end to serial killers and other sociopaths. They are just as prevalent as they have ever been.
While we do not know much about the human psyche and what really causes violence, I do believe that nurturing plays a big factor in determining whether or not the nature of the violent gene or tendency will surface and become part of the person's personality.
Think about it. If you have the violent gene, which is just waiting on the surface to be released into your personality, and the nurturing by your family and / or friends is badly done. Imagine if you are often physically and / or mentally abused constantly for a long time during your childhood. This would make you bitter and angry. You might lash out in retaliation against your abusers or you might make the abusive personality a part of your own because you believe that it is normal based on what you see around you in your environment.
Even people without the violent gene might become violent due to nurture alone. If they are repeatedly abused and see violence on a daily basis, it will seem like a normal behavior and they will take on the same trait themselves. However, I believe that, if you have the violent gene, then a negative nurturing will be even more of a reason for that person to become violent and her personality. It will be like a double whammy.
In closing, I believe that violence is a combination of nature and nurture.