The video looks consistent with that I've heard reported elsewhere: Protestors are not attacking people, only symbols of the Chinese government. The violence that has been perpetrated has been directed towards the protestors, e.g. the knife attack on a protestor, the fireworks shot at protestors (seen in the video) and various excesses of the police.
Every story has two sides and things on the other side - police officers bitten to bleed or eyes flashed by high power laser pointers. The sad thing is, we gotta take whatever the media feeds us:)
HKers are protesting because (for historical and economical reasons) they have been blessed by an incredible fortune, a fortune that's shaming away.
We could talk about reasons and sides, but that's misleading and unproductive. The sad reality is that China is a "Unitary one-party socialist republic", a bad euphemism for an undemocratic country ruled by a single dictatorial party.
There's a lot of misinformation surrounding the violence of protesters, but let's not forget that these officers are protecting the interests of a regime.
https://www.youtube.com/watch?v=rd1nW7Oa3mc