I am from Hong Kong and I want to share some of my specluations.
As answered from others, Hong Kong Police recruits from Hong Kong permanent residents.
For the second part of the question, I believe the police on the front line simply follow the order of the senior officers.
Violence from the police on 12 Jun, starting from 0:37
There was massive yet peaceful protest on that day while the authority blocked the citizens from entering the protest area of the LEGCO (Legislative Council) Building. Around 15:40, some people escalated their demonstration and began to force their way to there. Few of them threw bricks to the police and advanced with umbrella. The police then started to strike back. The video shows the police used tear gas against unarmed people (which I would say the majority of the protesters) and even beat up reporters.
An article by a witness of Siege of CITIC Tower, where the protest took place nearby
The witness stressed that the protesters being pushed back from the police already clogged outside the building and they were trying to enter to the building to escape. Even so, the police continued to fire tear gas to the crowd and cause great chaos.
On 21 Jun, due to the outrage against the police excessive force against protest on 12 Jun, people blocked all exits and roads of Police Headquarter since 11:00 till midnight of next day. Despite of such provoking act (from the police perspective), the police simply discharge their staff since noon and left some inside without taking action to drive out the crowd until the early morning of next day.
Encircling of Police Headquarter on 21 Jun
On 1 Jul, another large protest broke out again since noon. This time people equipped with safety gears (which were actually for construction purpose, simply to those on the people of above link) stormed the LEGCO building twice on that day. First one happened around 13:30. Police defended inside the building when people outside were trying to broke the glass wall of the building. Police took no action.
At 21:00, protesters broke the gate of the LEGCO Building and stormed into it. Despite of well preparation and equipment behind the gate, police again took no action and retreated.
Storming of LEGCO building on 1 Jul
To conclude, the police force is a well-trained force and strictly followed the order. From the above events, you may observe the police can ruthlessly beat any people including reporters or ignore the sabotage in front of them, depending on the orders. I am afraid if one day they receive the order of shooting Hong Kong citizens they may choose to follow the order as they already labeled those people as "rioters" who have to be eliminated by any means.
Statement of Junior Police Officers' Association