beyond the general idea that we shouldn't normalize gambling, betting on some real-life events is horrid. think about insider trading on a polymarket bet for someone's death.
You missed out the reason that it's horrid, which is that it is a plausibly-deniable way to crowdfund assassinations.
Or even worse what happens when people start gambling that someone won’t die today? It opens the door to crowdsourced hits with plausible deniability.