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.

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.