If this one incident makes people lose faith in the Hong Kong protests, it means their convictions weren't that strong. Whether or not it's justified, it pales in comparison to what's been done to the Hong Kong people by the government.
Furthermore, all these commenters saying that this was unjustified are coming from a position of privilege. You don't have to wake up tomorrow and walk down the street and worry about getting shot at. You don't have the specter of a totalitarian government haunting you each and every day. You don't have friends and acquaintances and family being jailed for no reason. You don't have your police force raping and straight up murdering. When none of these dangers are present in your life, you cannot understand what it's really like. You don't understand the totality of the situation, the desperation. The fear. The anger.
Once all those other conditions have been met in your life, and you manage to conduct yourselves according to your moral high-ground, then you can come here and call the Hong Kong protesters radical or uncivilized.
And besides, until other foreign governments are actually willing to risk their necks and intervene in the Hong Kong situation, I don't believe that the citizens have any right to condemn what's being done by an oppressed people.
Would you condemn the Jews if they fought back against the Nazis in WWII, knowing what would eventually happen? Think about that for a moment, and let it sink in. The moral platitudes are exhausting, especially when no one actually helps. And that's the thing; wax poetic all you want, unless you actually urge your government to action, it doesn't help. But that's just the way it goes, most people will express sympathy and move on.
And look, that's okay. I'm not forcing anyone and saying that they have to pack up their bags and help. But it's a matter of integrity and putting your money where your mouth is. People don't have the right to condemn what Hong Kongers are doing unless they are actively making strides towards an alternative solution (ie. They are doing something peacefully or pushing forward government action). I don't know if hypocrisy is the right word, but it's around there.