How about just holding cops themselves personally responsible when they choose to use deadly force when it wasn't warranted, or when they choose to injure a civilian when it wasn't warranted...instead of just leaving the community that employed him to pay ALL of the bills/awards/damages due to his negligence...?
Cops are already personally responsible for every action they take. They can be sued or prosecuted. In civil matters, folks just sue the agency because of deep pockets. Being a cop does not give LEOs much if any relief of personal (legal) accountability.
If a cop pulls the trigger on someone, the internal murder investigation starts immediately. It's just not called that. The circumstances under which that act is allowed are quite strict and well-defined, both by the letter and previous court interpretations.
In the end, it is the jury who decides what is reasonable, and not anyone else. This odd arrangement, the best we've been able to come up with in 5000 years, is precarious at best. But consider the alternatives.
Even if government prosecutors refuse to prosecute, that does not protect the accused completely.