I believe that while protester's violence may very well be morally justified (I don't have enough data to judge, but I can easily imagine this case), I don't think that it can be effective to achieve protester's political goals. Violent protest is only effective when the violence reaches it's logical conclusion and opposing force simply withdraws or surrenders. As was the case in Ukraine in 2014, or in USSR in 1991, it doesn't even need to be a LOT of violence - just enough for the opposite side to get completely demoralized. But China's police and military are not only numerous, well-trained and well-funded - they're also very highly motivated and believe (I think, mistakenly, but truth of their belief is irrelevant) in their cause.
So, the only way for protesters to succeed is to make China look bad: and generally, peaceful protesters are much more effective at creating positive sentiment than armed ones.