Well depends what their intentions are, they might confiscate all the farms to launch the attack and destroy btc to be able to force on public their own "cryptocurrency". In this way, they would have the best of both worlds, the biggest competitor would be gone, and everyone in china would have no other choice but to use their centralized government shitcoin. It's actually scary what countries as powerful as china can do to bitcoin.
It isn't necessary for them to launch a 51% attack against Bitcoin at all. Launching a 51% attack against Bitcoin wouldn't actually kill off Bitcoin but it could just spin off another fork of Bitcoin for the community to use; one that is ASIC resistant. In addition, there isn't actually any incentive to take the efforts to amass such a huge amount of computing power from the ASIC farms to attack Bitcoin. If they want to push for their own currency, they could easily just continue what they're doing right now, to block off Bitcoin related sites using their firewall and promote their own online currency.
The motivation behind a 51% attack for a state attacker isn't huge given that the gains is largely intrinsic.