A reenvisioning of the problem
You don't affect the votes of other people. With respect to your decision, their votes are fixed and unchanging.
We can therefore abstract away from voting to get down to only the parts of the hypothetical that pertain to your decision. So let's try:
**Scenario 1: **
There is a group of people. We don't know how big it is. It could be a few tens of millions of people. It could even be almost all of the world's population. And depending on how big the group is, what happens to them is going to be different.
If the group is larger than 50% of the world population, nothing happens. If the group is exactly equal to or less than 50% of the population, all of them die.
We can't tell you how big the group is.
The question to you is, do you join the group. So do you join?
**Scenario 2:**
Actually, I think we can abstract it further. We don't even need the group, just a giant roulette wheel. It has all numbers from 0 to 8B. Each number is represented at least once, but some numbers repeat, and are thus more likely to be chosen than others. You can't get a good look at the exact distribution, so you don't know exactly how likely each number is.
Every number greater than half of 8B is black. Every number less than half of 8B is red. They are mixed randomly
They are going to roll the roulette wheel and if it's red, the number of people selected dies. If it's black, everyone lives.
You have a choice. You can change the color of however many pockets are exactly "4,000,000,000". Right now they're red. You can choose to change them black. You don't know how many such pockets there are, but there is at least one. So now if pocket "4,000,000,000" comes up, everyone lives, whereas before 4B would die.
But if you make that change to those pockets, and any red pocket comes up, you add one more person to the number who will be killed and that person is you.
So do you change the color of the number 4B pocket, and gamble your life on the roulette wheel, or just let the wheel roll without you?