What does, or ought to, separate us then is our highly developed sense of morality, a primal understanding of good and bad, of right and wrong, of what it means to suffer not only our own pain—something anything with a rudimentary nervous system can do—but also the pain of others. That quality is the distilled essence of what it means to be human. Why it's an essence that so often spoils, no one can say.
For grossly imperfect creatures like us, morality may be the steepest of all developmental mountains. Our opposable thumbs and big brains gave us the tools to dominate the planet, but wisdom comes more slowly than physical hardware. We surely have a lot of killing and savagery ahead of us before we fully civilize ourselves. The hope—a realistic one, perhaps—is that the struggles still to come are fewer than those left behind.