The problem of police brutality is a very complex and layered problem. I've been aware of it all the way back to the 90s having dealt with some zealous officers. While I won't deny racism is part of the greater problem and certainly does not help matters, you have many other facets to this problem.
• A job which gives a lot of power into the hand of certain individuals and makes it very tempting to abuse said power and tends to attracts certain negative personality types.
• A police force which has become overly militarized, coupled with improper training.
• A group of people who view themselves as the elite of civilian society and have a strong union (the fraternity of policemen) who will ALWAYS cover for their own.
Also, let us not forget that police officers are not robots, and are human beings just like us. They perform a very dangerous and stressful job, and at the end of the day, they just want to be able to get back safely home to their loved ones. so I guess, to the list of problems you could also add.
• Inadequate psychological support.