I never got the impression that most police officers joined the force so they could protect the rich and shoot down the poor. I don't think that it's necessarily human nature to seek out a career with the exclusive intent to gun people down. I am not saying that's true for all officers, but I do think that most don't WANT to kill, they don't WANT to deal with everything that comes after killing someone, but they have to. Their job is on the front lines dealing with a lot of shady and scummy people. I often think that sometimes if they don't learn to cope with that appropriately they end up in situations where they become so hyped up and over the top that there's a lot of over-reactive behaviour happening.
I also believe when you are in a career like that your fellow police officers become your brothers and they stick together no matter what. I also think that there is a hive mentality that begins to develop. If you get one corrupt officer it stands to follow that others will share in that corruption. Corruption is a disease that spreads and some people actively participate while it sneaks up on others.
I think problems started happening when the corruption started from the top down. Over time, it filters down through the ranks and if you're leading by example and others follow in your food steps then naturally that sort of corrupt abuse of power spreads like wildfire.
There is accountability but perhaps not in the way that many would like. There also seems to be this mentality among those not in the force that every killing by a police officer is a senseless one. This is why those cameras are necessary and should be MANDATORY. In most cases, we're dealing with circumstantial evidence with some he-said, she-said and cell phone shots that only capture part of the story.
These days, I do believe that the police forces have become much more militarized. They are there to "protect" people to a certain degree but mostly they are re-active as opposed to pro-active. There is no longer a healthy relationship between people and the police. They are now separate entities who work against each other for a common goal which should be peace, but often ends up only being resentment and violence and disdain.
I do believe that they are subject to their superiors. Combine the Top-Down power model and wanting to keep your job and not be ostracized or abused or whatever, you end up doing what you're told and piggy back on those few individuals who DID get into the force to brutalize and abuse power. As long as you can follow the Alpha Dog who wants to rip everyone to shreds, you can shrug off a bit of accountability and responsibility.
It's not as simple as "X Police officer Shot X Person in cold murderous blood!" It never will be that simple. We're not dealing with police forces that are comprised 100% of Psychopaths here. There is a bigger, deep issue at hand that can't be resolved until you start looking for the source of the problem.