While I think murder is a bit outside the in-built nature of human beings, I absolutely believe that violence in general is one of the core human behaviors. People resort to violence very often, and very quickly. Sure, there's incentive to get the other guy or girl to back down without expending the energy of physically attacking them, but I'm not limiting my definition of violence to only be physical. Psychological violence is half of the infantry's job, anyways. I also offer another[0] interesting story about this fact (that most humans will not voluntarily murder another human being).

[0]: http://www.historynet.com/men-against-fire-how-many-soldiers...