Heres some random ideas,
If everything is natural, then nothing is unnatural. And if nothing lies outside of our universe, then everything outside of the universe is unnatural.
Nature, in his definition, is not relative, taking a dump is just as natural as committing suicide. Darwin said that the goal of every species is to pass on their genes. If that's true then committing suicide isn't natural because you are going against what nature wants you to do.
I think the reason you're friend believes that violence is natural is so that he doesn't have to reflect upon humanities poor decisions. I think he's being ignorant.