Stock markets also want to keep executives honest. When the insider can affect the outcome, it creates bad motives. They don't want the CEO selling a bunch of puts, then deliberately tanking the stock. Not for the other bettors, but because the institution is about business.

Prediction markets are doing a bit of that. Some won't take bets on an assassination.

You can bet on assassination. There are polymarket prediction markets for leaders of many countries where you can bet on if they will cease to be the leader by X date, for any reason.

If they get assassinated, those markets will resolve to yes. At least the rules don't specifically exclude that.