Any cop who stood by and watched a murder take place and did nothing is as guilty as hell in my book. The fact that they avoided charges is meaningless. They stood by and watched a man be beaten to death, they are a disgrace and have no business in any position of authority.
The policemen did not stand by and watch the suspect beaten to death, simply because the man died days later... when the police were not there watching him die.
The police took him in about 2 minutes. Standing there doing nothing would be much longer than that.
The fact that the other four policemen are not charged is because America is a nation of law. There are people let go all the time who were at the scene of a crime, even from in the car or home of the offense taking place by another.
They art also criminally liable under 18 US Code 241/242.