Let's set the existence-of-God issue aside for a later volume, and just stipulate that in some way, self-replicating organisms came into existence on this planet and immediately began trying to get rid of each other, either by spamming their environments with rough copies of themselves, or by more direct means which hardly need to be belabored. Most of them failed, and their genetic legacy was erased from the universe forever, but a few found some way to survive and to propagate.
Neal StephensonSee, the world is full of things more powerful than us. But if you know how to catch a ride, you can go places.
Neal StephensonEven the best of us have certain psychological mechanisms that can suddenly kick in and turn us into monsters.
Neal Stephenson