People have successfully won cases of self-defense against police, they are rare though. Firstly it would vary greatly by state, as not all states give you the right to defend other people. The next problem you will face is not just proving the cops were going to kill the person, but that they were not justified in those actions. You finally face the most important part, it is illegal to resist a arrest (even if the arrest is in error by a uniformed officer).

So, if a person was wearing a police uniform, walked into a mall and started shooting randomly, yes you could shot them. If they were trying to arrest a person for say a murder that occurred in front of them and they were resisting, and you come around the corner and tried to break it up, you would go to jail.

In a case like Floyd's, it would probably come down to if he lived or not oddly enough. Him dying would act as proof that you were trying to defend, though you could be charged with his death via felony murder. Him living though would act as proof by a prosecutor that he was in no danger. Oddly enough, you saving his life would be the very thing that could get you sent to jail.

Basically, when it comes to self defense you should be focused on yourself and your family. As I say, my gun's job is to protect me, not to protect you.