That's a very good point
Time and again cops who try to restrain the excesses of a colleague get ostracized, disciplined, fired and even put into harms way.
The culture and incentives are very much against the good cops prevailing.
From a sociological point of view it's very interesting that private sector gangs have a range of incentives which tend to favour moderation and peaceful outcomes (violence is expensive, and most people avoid engaging with dangerous individuals and groups - it's bad for business for both ordinary local people and for gangs).
but for the state sector cops, the costs of excessive violence are not borne by the individuals indulging in the excesses, they are socialized onto the tax payers, and the cops operate as a monopoly - their clients don't get much of a choice of alternative suppliers.
When it becomes clear to sufficeint people that cops are more of a threat than a benefit, people seek alternative arrangements, whether from neighbourhood organizations, or by forming gangs or contracting with gangs.
With the inner city gangs, the depredations of the police result in a very high turnover of people, and there is therefore insufficient time for the sounder individuals and groups to prevail - hence the constant power struggles to succeed the latest guy who got shot or locked up.
