I think you have a distorted view of what the role of Police is in a society.
It's authoritarian, deplorable, contrary to the common good of the people and in every way wretched.
With that said considering the current movement and public enablement of the PRC and Winnie the Pooh's iron fist, nothing you've said or what we have seen in Hong Kong is a surprise. Never the less, the entire situation is an incredible backwards step for common kind.
I don't doubt that the situation is complex, but lets get the facts straight.
They appear to have commercial ties to Shenzhen across the border. https://en.wikipedia.org/wiki/2015_Yuen_Long_protest
Historic genocidal wars are probably still remembered. https://en.wikipedia.org/wiki/Punti-Hakka_Clan_Wars
That is not what happened. They took it as a challenge after they were beaten up. No one did anything to Yuen Long, and they decide to beat any everyone, from Child to Pregnant Woman on the Western MTR Train.