Why would the opposing side of that exact same bet allow themselves to be fleeced of all that money for free?
The opposing side is getting paid, not getting fleeced.
Asymmetry of information. Fixed bets depend entirely on a small enough group with the ability to influence the outcome, keeping their mouths shut.
You're asking why someone hiring a hitman would be willing to part with their money?
Because that's what money is for, to purchase things, like hitmen (apparently).