In this article on Scientific American, Eric Michael Johnson looks for an explanation as to why the United States, one of the wealthiest countries of the world, is also one of the deadliest. One of the answers seems to be not related to lack of religion or violent games, but that aggressive behaviour is part of our primitive nature.
Just as it is with so many other issues in our species -- infanticide, sexual coercion, or collective violence -- I believe we can most successfully pinpoint the broad patterns in our behavior by thinking like a primate. In most social primates (including humans) males frequently engage in aggressive competition over status with other males in their group, maiming and sometimes even killing in the process. Naturally, there is a good reason for this: sex.
In chimpanzees, for example, Cristina Gomes and Christophe Boesch have documented that the two most common reasons females will choose to have sex with a male is if that male had shared meat with them in the past or if they were high-ranking. In some species, such as hamadryas baboons, the male obsession with status has taken an extreme form. Males of this species are nearly twice the size of females because, over evolutionary time, those males that were slightly larger than others had a competitive advantage and passed on more copies of their genes as a result. Of course, all of this male-male aggression comes at a price.