Taboos around violence for political are one of the crucial building blocks for a functioning democracy. If those taboos are broken, even for a good cause, you set a precedence that violence works. And the next cause won’t be as good. One only has to look at the lessons of the Roman Revolution that started with the murder of Grachus, and ended with an Emperor who everyone acclaimed as they were so tired of the bloodshed.
Tell that to the police. The current "don't use violence" rhetoric is cleverly being aimed at the protestors and seems to be giving a consistent pass to the other side's behavior. In many cases, these guys are suited up like shock troops, visibly excited and ready to bust skulls, and when they're unleashed they are going to find skulls (peaceful or not) to bust. This is a system that only knows how to use violent escalation to solve problems, and lo and behold, they're out there bringing on the violence. The protests are about police brutality, and the police are coming in and using the only tool they know: brutality. But it's OK because someone somewhere else is burning down a Target?
> To use it is effective acknowledgement that you either want to destroy the democracy or do not believe that it actually exists on the moment.
From your wording you seem to be agreeing that the police also "want to destroy the democracy".