A national law would work best but even state laws would be effective.
I consistently and strongly disagree with this. The police are a manifestation of our society. When they do bad things they do so in our name - whether or not you want them to do it. We have created a law enforcement structure that empowers these abuses to happen. Transferring the cost to them for acting the way society wants them to act is not the answer. We bear that responsibility and should bear that cost.
This _maybe_ works in some abstract way, but each police officer is autonomous and has the leeway to act any way they want.
I have autonomy as well, so I could commit a crime.
Let's say that I grew up poor, and the only opportunity in my neighborhood to make any substantial amount of money was to sell drugs. If I choose to sell drugs and get caught I go to jail.
Now at this point, we can say that we have created a societal structure that limits my opportunities (no economic development in certain areas) and empowered these drug sales (by making the drugs illegal thus increasing their value) to happen, but that does not somehow shift the blame from me to society.
I would do my prison sentence and try to move on with my life.
This apologetic stance for the abuses of the police is harmful.
Police officers are people and should have at least as many responsibilities and liabilities in the view of the law as any ordinary citizen.
In fact, as they are charged with upholding our laws, they should be held to a higher standard than an ordinary citizen. If a police officer commits a crime, then they have committed that crime _and_ they have abused the trust placed in them by the state, county, etc that they work for.