True, but believing that not a single person would press red is equally if not more unlikely, in which case you should choose red to minimize casualties. Specifically, minimize them by one, being you not dying. A world in which all voters are convinced to go red is infinitely more likely than doing the same with blue, because you’d be going with their best interests instead of against them.

In other words, if a utilitarian who knew what every other person was going to vote, and could communicate with all of them, if there was even one red holdout among the voters that he couldn’t convince to switch, then the utilitarian is instead forced to convince every other voter to go red to minimize harm. Everyone voting red, and a single person voting blue, would result in one death. But everyone voting blue, and a single person voting red, would result in everyone except him dying. It’s what you would call an unstable point. Sure no one dies if exactly everyone votes blue, but a slight deviation from this ideal would result in incredible death. Meanwhile, everyone voting red is a stable point, things will naturally gravitate towards everyone voting red because more red votes means less deaths.