It hasn't happened in this case. Your example has causality reversed. Khamenei wasn't killed because there was a prediction market about the political leadership of Iran.
It is entirely possible that there was causality in the forward direction if the bettor had inside information about the assassination operation beforehand.
I knew my previous employer was getting acquired before the markets did -- I had inside information -- but I had no way to make the deal go through.
Revealing inside information through prediction markets is mostly fine. You may think people shouldn't need a financial incentive to do so but clearly they do, and clearly other people are willing to put up with that money for it.