The video. Pretty clear he's swinging something that looks like a pipe or a wooden stick. Not the full context, and I imagine the police have things to answer for. But swinging a stick/pipe at an armed person isn't wise.

https://www.facebook.com/hkucampustv/videos/542398913195804/...

While I would expect repercussions when swinging at police, I think being physically present in confrontation shows a level of commitment that a weaker protest movement might not show. Sure, if you die you aren't going to be protesting any longer, but that doesn't mean it has no strategic value as perhaps the only option a protester might see is violent escalation.