Well its hard to crush someone's face in 2.4 secs which is the average time of a physical connection and shooting. They tasered him, but it didn't work. so that's another layer. I read that he recently got out of 15 yrs in prison for armed robbery, and that the call was for a robbery. Of course, it definitely could have been as you suggest, because they were eye sores and the 911 call was a fake... time will tell.
You are correct about the experience part. The cop that was beating the homeless man up was the least experienced and yet was doing most of the work.