Murder

Innate.

It is used when all learned ways to handle things are tried and found not working. After doing it once, you will probably learn, that murder can be one way to handle difficult things.

I think it's a combination of both.

Scientist already located a gene that's responsible for violent tendencies. However, studies have shown that some people with specific behavioral Genes (like depression and other mental conditions) get to actually be depressed and some didn't.

For depression to actually accrue, there should be a combination of both depression tendency (which is genetic) and a life situation that will allow an outburst (inner or outer stress, a shocking event, a lack of knowledge about how to talk about your feeling and so on) a lot of this is social rather then genetic. Also people who doesn't have the genetic caricaturists can develop depression due to life conditions and education and background.

In my opinion, homicide tendencies might be genetic (a part of human nature) but the actual act of murder is a learned trade (which is effected by education, psychological condition and other social factors).