Resource deprivation. Competition. For a number of major nations, foreign wars.
For those in any of the member states of the "Coalition of the Willing", the illegal and unjustified war of aggression against Iraq has resulted in as many as 600,000 deaths (Lancet). That's one for every 512 Americans.
And that's merely human deaths. If you include animals slaughtered or killed in the name of humans (for food or as a consequence of human activity) it's tremendous.
If we are going to discard the contextual meaning from above (people killing people) we should include the billions of yeast I baked alive the other day. And all the microorganisms I'm digesting incidentally.
How?