There's a branch of science called evolutionary biology that suggests many, if not most, of our behaviors come to us from our prehistoric ancestors. According to this line of thought, the reason we kill is because our ancestors killed. By killing, our ancestors removed rivals and ensured the survival of their offspring. In other words, we're violent because all the peaceful ancestors to humans were killed off by the violent ones. We've inherited our nature from our predecessors.
This view is by no means universal. Scientists from different disciplines have criticized evolutionary biology, saying that it oversimplifies human behavior and serves as a genetic excuse for bad behavior. While there is scientific consensus that the human brain is the product of evolution, there's a gap between those who think our brains are in Stone Age mode and those who say the brain is much more flexible than evolutionary biologists admit.
One counterargument to evolutionary biology states that our minds are adaptive and evolve far faster than evolutionary biology can explain. Differences in cultures around the world suggest there is no universal human nature -- the environment and our adaptation to it means that each culture has its own unique nature [source: Begley].
On a superficial level, it seems like the explanation for why we kill boils down to another nature-versus-nurture argument. The nature side suggests that we are inherently a violent species and it should come as no surprise that we sometimes kill one another. The nurture side says that we are an adaptive species and that our environments -- including everything from family structure to political influences -- shape our behaviors. The truth is probably that we're a product of both. Ignoring one set of influences while concentrating on the other is missing the story.
If we're the product of both inherited traits and environmental influences, what would give us the reason to kill? Many answers boil down to survival. In some cases, it's as simple as access to resources. Whether it's a conflict between two people or multiple nations, the reason to kill may be linked to the fact that one party wants what the other party possesses. That might motivate people to kill in order to take or protect those resources. The intellectual and emotional need for those resources is often greater than the reluctance to kill.
Not all violent conflicts are over resources, though. What else makes us kill?