Your logic is flawed. Why would the banks need money? The people running the show are the ones that are at risk.
well your is even more flawed then, because you started with saying "if banks turning on(which mean buying) 300peta of hash", i answer in the same way, why they need to do this if they don't need money??
also it doesn't matter what is their intention(even if they want only to kill bitcoin), they will lose money doing it, end of story