A few beatings with truncheons does not police brutality make.
You will see this in any social disorder/ riot situation.
Just wait until the mainland assert their authority. Then you will see just how effectively things can be brought back under control.
I teach 'a few' as meaning a small number, often three or four. I have seen videos of dozens of such beatings, and I haven't been closely following this.
The mainland has limited legal scope for asserting authority. It can certainly happen, but they are constitutionally unable to intervene unless the Hong Kong government requests their help because their own police forces are unable to restore order. They may opt to wade in hugger-mugger, but excessive brutality will cause commercial and capital flight from Hong Kong at a time of considerable economic instability. Any government has the option of abandoning the Rule of Law, but the consequences are frequently worse than the situation it seeks to alleviate.
So why do you think some Hong Kong residents have chosen to shoot themselves in the foot by instigating something they must have known would grow into this.
It didn't have to be so.
Because it's easier to stop the thin of the wedge than the fat end.
More generally, this is supposed to be a CONSPIRACY thread.
Has this situation been engineered or perpetuated by the mainland to reassert control over the (I keep on typing crown colony) territory? I doubt it, for the reasons I have given above. In the wake of the Tiananmen Square massacre, China underwent a plunge in overseas investment, was sanctioned by the U.S. and suffered an economic downturn and a freeze on the economic reforms that had brought China into the modern world.
The Chinese government wants no replay of this. If it gets one, it'll be the product of gross incompetence and the myopia of hubris.
Are there agents provocateur and undercover police attempting to infiltrate the protest groups? Of course. While there is some interest in the details of how they attempt this, there can be no surprise at all that it is taking place.