How to destroy Bitcoin (or any other PoW crypto currency with futures)
1) About a billion USD. Might work with less, more is better.
2) A lack of moral or a moral justification like "Bitcoin mining is a waste of resources and should be destroyed.".
3) Miners or supercomputers willing to rent out their data centres to you.
4) Knowledge about financial market, Bitcoin and some 10-100 friends/network.
First step: Buy about a billion dollars worth of Bitcoin while you at the same time sell Bitcoin futures for the same amount.
Second step: Rent hashing power for some $20 million per day (more than the combined hashing power of all crypto currencies) for a couple of days.
Third step: Start mining like crazy, but make sure that there is a "network error" so that the blocks you produce aren't broadcast.
Fourth step: Send your hundreds of thousands of Bitcoins to exchanges (or something) and exchange them for other assets. Of course you need to do this in an anonymous way. At the same time you make sure that your mining doesn't include most of these transactions.
Fifth step: Fix the "network problem" after some hours/day and reveal your blockchain that is now longer than the ones people/exchanges used when they accepted your Bitcoin. This blockchain is now the one that is valid and all transactions that were mined in the public blockchain are no longer valid.
Now exchanges will panic, start delisting Bitcoin and/or demanding that people wait for days to get their transactions confirmed. There might be a fork of Bitcoin a people demanding that your blockchain isn't valid because it didn't broadcast its blocks and so on. You will yet again keep on attacking the network to cause massive double spends.
Then you can buy back the Bitcoin futures that you sold for a great profit because the price crashes, and you also have the assets that you bought for your Bitcoins. Note that this might be profitable even without futures too, but as the Bitcoin price crashes, you might not get much for your Bitcoins the second time you want to use them.
So: Why wouldn't this work? Why hasn't it been done already? Or has it been done in a small scale and no one exposed it?