Protester shot in chest by live police round during Hong Kong protests

Saw some of the videos and reports, it is awful. One thing I don't understand - these cops are also from HK, correct? The protestors are fighting for the cops rights too, correct? If that is true, what is going on in the mind of a cop who is beating an unarmed protestor?

Or are these cops from mainland China?

cops are trained to view their own population as hostile everywhere