A government cop can NEVER claim self defense while on duty because their existence as a cop makes them the automatic aggressor in every situation they encounter.
You can't create a situation with aggression towards others and then reacte to the victims reaction to your aggression, in self defense.
I don't exactly agree with this, although it does point to a problem of over aggressive police with non violent citizens. A large part of the reason police exist or were created is to deal with violent citizens involved in any number of violent crimes. (robbery, rape, murder etc.) Police aggression is very likely self defense when dealing with these individuals.
The problem is the manner that police tactics have evolved in a significant portion of todays current police population. There are great cops out there. But there are also way to many, as caught on video time and again, who's idea of tactics is to jump into thuggish behavior when it isn't warranted with a non violent citizen. Good cops get their reputations tarnished due to the unwritten code of supporting other cops no matter what.
Most states do have laws stating that a citizen can resist and defend themselves against an illegal police action. Whatever that means today. The justice system isn't perfect because there is to much money involved. This potential source of funds has corrupted the system to a degree. The justice system certainly isn't the fair and just system that we were taught in school as children.
In the Brown affair, I see it as I have many of the others which have occurred. A thug got shot for being thuggish. I can't say that was wrong. But I will say it only happens to citizens who are thugs, not to todays individual police officers when unwarranted thuggish tactics are employed.
I really like the idea of police being mandated to carry cameras on their person which are on with any interaction with anyone. Where implemented, it has resulted in better, more professional police behavior among those police officers not inclined to display that trait.