Police in any countries will do their job by ensuring that there is public order. In protests that serve to disrupt daily lives of other people, such as in Hong Kong's case, police has no choice but to bring order back.
Protesters can sometimes be vicious. The organisers are cunning, deceptive and sometimes boundless by law and conscience. The purpose of the blockade facing Hong Kongers is to disrupt the lives of residents, workers, and daily activities. The protesters thrives to induce public anger at the government for failure to maintain order. The protesters also wanted to induce mismanagement by the government. The more missteps government makes, the better. These will be highlighted by media and relayed internationally.
Alas, police are always made the scapegoats in most cases. Police are the front line and as human beings, they will err emotionally. Again, to gain mileage, protesters have to induce a misstep by government machinery. To succeed, they need to taunt by insult and violent/unruly behaviour to get into the nerves of the police. In order to succeed, they need the help from the press (any press) on standby to capture it on video or pic. The protesters can afford to wait (a day, a week, and even months) for the bait to work. To have the police falling into the trap isn't difficult. The timing should be right.
Protesters are attacking the government at multiple angles, first disruption to daily/business activities, taint government image locally and internationally, incite anger and drive mob attack on government machineries, induce international intervention, and many more. If there are casualty to the mob of protesters (the pawns), it would bring success much much more closer to the organiser (or the leaders) behind the protest. A few word of regret and sympathy by the organiser would surely bring more support to the protest. That would be the most brilliant move on a "game of chess". That is also the reason why the protest was not done in a dull place such as in a national stadium or open field. What is the fun if the protest can't be escalated to a violent one?
That is where police comes into picture. Let the police be the scapegoat. Let them be the sole reason/trigger and most important an excuse to push up a notch to become violent.
If that happens, more property and lives will be destroyed. There is no loss for the leaders of the protest. They (politicians sidelined earlier) will gain plenty. Well, they can shed tears afterwards and blame the government for loss of lives (esp. the police for the cause). People will sympathise and misdirect the blame solely on the government. Hong Kong will receive letters and calls to "pressurise" the government. The protest will still be there disrupting daily activities.
What is the fun of a protest if it can't be escalated to being violent? That is the motivation to hold the protest to disrupt public order. Let the pawns drop. Let the police be scapegoat. Let the other non-participants suffer due to disruption, jams, loss of business, increased tension in the neighborhood, deteriorating safety level for their family, and uncontrolled emotional outbursts (from non-participants and protesters). The mastermind of the protest will gain more.
That is why these people lacked conscience. The tactic might work, or it might not (in Thailand's case, it worked for a while until military rule put a stop to it). Nothing ventured nothing gained. Loss of lives is nothing to them. The means to success is not important. The goal is more important.
My thought: violent/disrespectful protests won't work. Disrupting daily activities of other citizens is a crime. If the protest suppresses freedom of other citizens, the protesters automatically lose their rights to freedom of speech/expression. Not respecting the basic rights of other people disqualify the perpetrator his/her own rights, e.g. the rights to express by the protesters, which inevitably suppressed the basic rights of other people in the neighborhood is the most valid reason to revoke the protesters' rights (and can even land them in jail for the loss of property and lives caused by their action.
Just a thought.