How many videos, at this point, are out in public of police taking out tasers at routine, non-violent traffic stops? Of chemical spraying non-violent protestors? And of shooting unarmed people? The famous photo of the UC Davis photo casually spraying seated students at a campus protest makes the case more eloquently than I can: that was not a person in fear of their life, and we've created a system where that person avoided criminal prosecution (https://humansarefree.com/2014/04/the-cop-who-pepper-sprayed...). When that happens, it seems an offense to logic to reach to "well, our police are more violent because our criminals are more violent." It may be true, but it's outright offensive to their victims not to pause, first, at "...and because their violence goes unpunished." Pointing out the risk they are exposed to is less plausible when we see violence undertaken in non-violent situations, and where they do not see criminal prosecution for wildly disproportionate responses.
How many events have we learned of where unarmed people providing no violent resistance were killed by officers who didn't suspect violence? The death of Eric Garner in NYC involved an unarmed man, stopped by the police for selling single cigarettes, who was pinned down and choked to death, while his hands were up and he was wheezing and begging for air. Those officers were neither permanently removed from duty, nor prosecuted. It's difficult to interpret "slowly choked an unarmed and unresisting man to death" as "acting in response to fears of the violence possible in the situation." Situations like that make it highly unlikely that violent police are a simple correlation to violent criminals, because that would at least suggest that the police violence is constrained to violent and reasonably-possible-to-escalate-to-violence situations. Which isn't what we see riots breaking out over.