If the prediction market is for a non-trivial amount, it's likely someone is going to kill you in exchange for the money the prediction market offered them. The prediction market isn't acting as a prophet here, it's acting as a plausibly deniable murder for hire service and you are its victim.
The people "betting against" you dying just paid to have you killed.
Yes, OOP might have chosen a suboptimal example here. But for general newsworthy events, people aren’t going to be in positions to manually make them happen. And no person in a position to start a war would do it to affect a Polymarket bet.
Why would the opposing side of that exact same bet allow themselves to be fleeced of all that money for free?
Exactly, these markets exist in the real world, so as their size and use increases, the more likely the odds will influence real world events. Look at sports betting for a much smaller example. Match fixing is known. Electricity markets are gamed for individual profits at the detriment to everyone and the stability of the system, even with regulators trying to keep things stable. Enough "Market for all the things" already..