A striking fact about cases where police are caught on video murdering someone is that the officers involved often have a history of racist or violent behavior. This is true more generally: "A few bad apples" explain a high fraction of racial bias: https://dataspace.princeton.edu/jspui/handle/88435/dsp01z890rw746

How about incentivized, anonymous mechanisms for law enforcement officials to report on racist or violent peers before they murder someone? If multiple reports accumulate, officers are investigated -- if fired, salary for the next year is split among the anonymous reporters.

I think Heather Sarsons is (or was) working on a similar idea for sexual harassment -- people file anonymous reports, and if multiple reports show up for the same person, an investigation is triggered.

The investigators should be offsite officials, ideally with no connection to the department or officers they are investigating. They would interview everyone who worked with the offending officer (and need not even know the identity of the anonymous reporters).

Another possibility would be assigning liability based on these reports. For example, the chief of police has discretion about whether to fire X or not. However, if X is later sued for misconduct after having been flagged, the chief of police shares liability.

The liability point applies more generally -- give the people in charge a stronger incentive to identify and fire problem officers. If they fail to do so and subsequent investigations suggest they should have, they should lose their job and face financial and legal penalties.