But it is the fact that China currently controls the majority of hashes although it's far from 51% attack it could be a problem later on. However, if China really does harsh law enforcement regarding cryptocurrencies or even banned it with punishment for those who break the law. We could potentially lose a major hashes power However it will then adjust automatically after 2016 blocks and become profitable again for small miners to mine bitcoin. Actually there's nothing really need to worry about as of now and to be clear, China didn't control Bitcoin but a price manipulation with massive capital could still happen.
A 51% attack would leave Bitcoin completely worthless and anyone who has made investments in Bitcoin (including the large-scale miners) would try their best to avoid that. So I don't think that the mining pools would come together and launch a 51% attack, because it will be self-defeating for them. Only the government agencies would try something like that.