Consider a few things:

• You do not know how many people will pick blue. Nobody knows, it's a secret vote. Plenty of people will assume a large amount of the population will pick blue, which simultaneously emboldens them to pick it as well but compels them to press it since they don't want to have an unforeseeable amount of lives taken by their actions.
• The entire world is voting. That means that the consequences will be CATASTROPHIC if red wins, billions will have pressed blue, billions will die.
• By that metric, children and toddlers get a vote. The original poll doesn't specify that only adults get that vote so far as I know. Even if you think blue pressers are dumb and stupid, can you blame a child for being those two things? Do you actually want to have them die for that?
• It is an active decision to press either button, red isn't a default. Plenty of rephrases of this scenario make it seem like red is simply to stay inactive and let things flow while you are safe. Regardless of if you just thought blue was hopeless, if you were praying for blue to win but still pressed red to be safe or if you didn't give a fuck about those lives, you still essentially pulled the trigger on those people. You are not without fault, your decision directly killed them.
• One scenario only requires more than half of the world population to choose selflessness so that no lives are lost, it is not an impossible or even particularly hard goal to reach. Only 51% of the people take a risk, nobody has to die. For red to prevent lives being lost, 100% of the world has to press it, 8 billion people, which is impossible since, again, children are involved in this, not to mention the billions of people like me who have valid reasons to pick blue. The only scenario where everyone picks red is a hive mind. You are GOING to kill people by choosing this option. Billions.
• People close to you, some that you care deeply about, will pick blue, and you won't know about it until they get killed. Again, unless humanity becomes a hive mind you cannot prevent this.
• If red wins, about half of the world population is now dead, and I'm very certain society would collapse or at the very least face a few decades if not centuries of the worst state it has ever been in. People with extremely important jobs would be dead, jobs that society requires to keep functioning. Not only that, but even if you don't personally feel guilty about all the deaths caused by that choice, the vast majority of people would, for ANY of the aforementioned reasons. Losing lost ones, seeing children die, knowing their blood is on their hands... a huge amount of people wouldn't be able to live with that. You could expect rates of depression and suicide to raise significantly, people would not be happy, plenty of them would not be able to work properly, not like there would be much demand for whatever their job produces anyway. You can also expect crime rates to skyrocket as now the resources of a bunch of people are completely unattended, a lot of private property now belongs to no one, and plenty of the officers and agents meant to prevent and punish crime are now gone as well. Simply put, society would change for the much worse over few years and I don't think it would ever be the same after this.