@J Ambler,
Yes, my thread is brought back to life!
I dislike the idea of moral absolutes (killing is wrong), because the idea of pushing the button revolves around the idea that it is right to kill to save. So yes, while it is still killing, so what? It's killing for a moral purpose.
Okay, so let's break my certainty a bit. Yes perhaps pushing the button to save humanity would not work, and they might all die anyways. 50-50 chance on either.
Our choices are between:
1) Killing twenty million --> May save six billion and stop human extinction OR human extinction may be inevitable anyways. Total deaths: 50 %twenty million / 50% six billion.
2) Killing no one --> Human extinction is inevitable, six billion die. Total deaths: 100% six billion.
So even if pushing the button might not work, if it doesn't the same amount of people still die, and if it does many less will die. So statistically, from a consequentialist point of view, the moral choice would still be to push the button.