My theory on why the police need to use some level of brutality is because a mob mentality is an animal. It's an amorphous mammalian manifestation that is afraid of loud noises, pain, and losing it's ability to breath easily. So they use counterpart tools to control it, because when this animal cant be controlled, the whole city could be be razed in flames. (As an example, https://en.wikipedia.org/wiki/1992_Los_Angeles_riots#Destruc... )
Im not excusing shitty behavior, but it's worth considering there are millions of events going on in these protests, and all the excess force ones fit in a bullet list on my monitor. Also, in many of the listed events, the cops told people to move away or go back inside repeatedly before taking action. From their perspective, keeping battle formation is necessary to success. And lawfully, you are supposed to comply with this demand.
Just things to consider. I hope I'm not gut-reaction-down voted for taking the middle ground.