The state insists on a monopoly on using violence, except in very limited circumstances,  and police are the agents assigned to carry out that violence. They should be scrutinized each time they do so, because they are obligated by law only to use deadly force where their own or other civilian lives are in immediate danger. 

It's a go-to strategy from the playbook of dictators and police states to use ordinary cops to stifle dissent and impose fear. 

Public accountability is the best protection police have. Cops believed to be acting lawfully will have the respect and cooperation of the vast majority of citizens. The "Blue Wall" which shields police wrongdoing is the fastest way to squander that respect and cooperation.