Kelly Thomas was severely beaten by police officers on July 5th, 2011. He died of his injuries five days later. More details, along with videos related to the incident, can be found here and here. Fair warning - the images and video are NSFS (not safe for soul).
It's natural to wonder how something like this could happen. It's also natural to conclude that the officers involved are terrible people. What they did was inexcusable, and they should be held accountable. But I don't think the core problem is that the officers are terrible people. I think the problem was that they were police officers.
Maybe I should clarify that a bit. Lord Acton's statement that "power tends to corrupt" isn't limited to just statesmen. Police officers have power too, and the people who enter the police force are no less prone to corruption than anyone else. Had they never entered the police force, and instead taken up careers as dental hygienists, they likely would have gone their whole lives without engaging in any activity more violent than working on cavities.
I doubt that these men were evil people who joined the police force because they wanted to lord over their fellow man. I think we all overestimate how well we would behave in such circumstances. The Stanford prison experiment comes to mind. In his excellent book The Lucifer Effect, Phil Zimbardo makes an interesting observation about the students who participated in the experiment. Before it began, they were interviewed and asked if they would prefer to be one of the prisoners, or one of the guards. Every single one of them preferred the role of prisoner. They were all hippies, anti-authoritarians who held police in disdain. But, somebody had to play the role of prison guard. And when you gave these anti-authority hippies a fake uniform and tell them they are in charge of a fake prison, they quickly became so cruel and sadistic to their fellow students that the planned two week experiment had to be cut short after six days.