Police violence is regulated and codified. It’s a monopoly, so there is little need to do a ‘race to the bottom’ between different police forces (for instance), as long as they can win against folks like criminals.

In a black market, violence is a competitive advantage in many cases. The most violent has the most ‘authority’ (can assert the most control), so in a competitive environment there are strong incentives to be the most violent. Too violent for other players to want to mess with.

It’s why cartels, warlords, etc. end up becoming so terrible.