The problem with considering whether violent behavior is ‘born’ or ‘raised’ is that it assumes all violence is the same and must have similar causes.
Not all violent behavior comes from consistently violent people. There are cases, for example, where people become aggressive in the aftermath of a traumatic event, under acute stress or during a depressive episode. A teenager who is aggressive at school might calm down later in life. These are episodes of violence behavior, but necessarily chronic patterns.
On the physical side, people can sometimes experience a change in personality after brain injury with an increase in aggressive behavior, as can some dementia sufferers. This suggests that physical mechanisms are involved in some cases, but not always from birth.
There’s also the corrupting influence of power and the bloodlust of conflict. Studies have shown that people given excessive authority over others can sometimes go on to use it sadistically. Think of history’s awful dictators and massacres. Previously ‘normal’ people are quite capable of violent atrocities when a lethal mob mentality sets in.
Early experiences can, of course, produce more ingrained patterns. People who have grown up in a violent home might model, or adopt, the aggressive behavior of their parents. But aggression can also stem from a lack of the skills needed to manage angry feelings constructively. In other words, it’s not what they’ve learned, it’s what they haven’t learned. When discipline is overindulgent or inconsistent, children may find that dramatic displays get them what they want, and they never find other ways of soothing their frustrations.
So is violence born or raised? I think these are only two of many, many possible factors.
