The discussion around this is great. Here is my conclusion: Tell everyone to vote blue, but vote red.
I can be altruistic, but I can't bet my life on other people being altruistic. A lot of the people who swear you must vote blue would change their minds when faced with the possibility of death. They would rationalize a million different ways. And they'd know that everyone else was doing the same. Voting blue would collapse.
If people died, every single person alive would share any guilt (except this wasn't our idea!) It doesn't matter what we could have done collectively. Every individual would know that had they chosen differently, the only change would have been their death.
Therefore, red voters, I absolve you!
Everyone in the world has to take a private vote by pressing a red or blue button. If more than 50% of people press the blue button, everyone survives. If less than 50% of people press the blue button, only people who pressed the red button survive. Which button would you press?