Killing someone doesn't change whether or not they die, only when.

The only thing that determines whether or not someone will die is whether or not they were born. Therefore, the only cause of death is birth.

The only case in which murder changes the amount of people who will die is if a person who would've otherwise had children is killed. By preventing birth it prevents death. In other words, murder can only decrease the amount of people who will die or keep it the same, but never increase it.