I would choose 'natural deaths' as the leading cause of death. Natural deaths would include all women and children, whereas deaths by genocide and war would take in a larger amount of adult males than anyone else.
But then again, "natural" death is a somewhat simplistic cop out - how many have died from malnutrition or starvation, while others munched down in abundance? How many have died from the malady of disease, while others knew how to avoid such things? How many are dying today because of ignorance in nutrition or general health principles? How many writers continue over indulging in booze, knowing that alcoholism will shorten their lives?
But ultimately, Mother Nature is by far the greatest taker of life than mankind could ever think of taking....her count against, say Hitler or Pol Pot, would dwarf any of them. Mother can be a real bitch when she wants to.