But if the people in power use force to prevent a legitimate discussion then the only avenue left is revolution. There is no legitimate way for the people to involve in discussion, the HK protesters want democracy, the Chinese government doesn't. What alternative do the HK citizens have apart from not be part of China?

Edit: further there is no way to express dissent in China, the HKers are taking their life in their hands by expressing dissent. I can think of no time this happens in the west.