why is that
were the 5 black cops that beat this guy to death not criminals?
No, they were law enforcement officers abusing their authority. While this is clearly a rare, extreme example, milder examples are not all that uncommon, and are a matter of controversy.
Police officers are generally not necessarily the brightest, most highly educated, or best paid. They have to deal with the worst specimens of humanity on a daily basis and with the best of us on our worst days. They are subject to all kinds of verbal and emotional abuse.
It's not surprising if some develop an attitude and begin to view the public as subjects or suspects and potential threats.
Some also think they have the authority to stop someone, demand ID, etc. based on flimsy or absent probable cause.
Check out first amendment audit videos at Facebook or YouTube. Yes, those guys are intentionally instigating and provoking. I don't recommend trying that at home.