> What can you do about this? Pretty much nothing.
A lot. And what you can do depends who you are.
Hongkongers can keep doing what they are doing. I'm sorry, but killing everyone in HK is not an option for China. Not only would it start a global war, as genocides tend to do, it would create unrest in the mainland, and frankly people are a resource. You can't govern the dead. I frequently hear the arguments from Americans "well you can't fight the military". Except that's been done for centuries, caused many countries to come into existence (including, ironically, America), and caused many countries to have large economic pains (cough Britain fighting America cough, Vietnam, Iraq, etc). That idea just doesn't match history, because of one simple thing "you can't govern the dead". People are a resource that governments need. You have to ask yourself "why hasn't China brought the military in already?" They are flexing, but there's good reasons they haven't brought in their military.
In fact, they want you to believe nothing can be done. They want you to give up. (I'm not just talking to hongkongers here)
From the Western side there's also a lot we can do. There's a huge amount of economic pressure we can put on China (for now, the still need us more that we need them). We can grant asylum to refugees. There's a lot of political pressure that can happen. There's also a lot of military pressure that can happen behind the scenes (I wouldn't be surprised if this already exists in some way, because China certainly does it to us), be that propoganda, cyber, or training.
There's even more that can be done. But I find this notion of nothing can be done as ridiculous and backwards. It is giving up hope and ignoring both history and reality.