The issue is the combined risk of insider trading coupled with the bias of disaster-centric betting, or at least event-centric betting. This means if you have the means to create an “out of the ordinary” event you have a strong incentive to make it happen and to bet on it. These must be controllable events, so not natural or complex systems. On the gentler side it would be sports fixing, which has always existed. On the worse side it would be causing war, making economic decisions that will impact many, betting on people death and so on. These kind of things are seemingly already happening to a certain degree.