Well, you could be a government whose goal was to destroy the cryptocurrency.

Are there no markets with put options on coins? Also couldn't this be used to prop up a competitor coin.

There are probably lots of ways to make money of destroying confidence in a specific coin.

Shorting the coin wouldn't work well because you have to take a long position at least temporarily in order to execute the attack; you would have to borrow 51% of the supply and also a bunch more coins that you'd then sell short, enough to more than compensate for the cost of borrowing the 51% and the risk of failure, and that would get more-than-proportionally expensive. Seems hard to make the math pencil out.

Taking a long position in a competing coin could maybe work but you'd have to be really sure that it would go up, instead of going down due to decreased confidence in the broader altcoin ecosystem.