Whether the policeman should have been accountable or not is irrelevant unfortunately.
As the judge said, many of the witness statements were exaggerated or otherwise unreliable and therefore introduced reasonable doubt. So even if the policeman had shot the kid in the back of the head execution stylee, he would never be convicted.
The US is a bit of a mixed bag, the best and worst of systems... Everyone has the same opportunities -a very positive ethos. But due to the way US communities are funded. This essentially means that families who did well 1-200 years ago and continued to thrive hold most of the power, be that on a town/community level or a County level or a State level. The middle and working classes can do well in their lives, and many do, regardless of colour.
The poorer sections of society, inspite of having the same rights and opportunities, have no influence because they don't/can't financially contribute to their communities.
There are philanthropists who do alot for the whole community and there are those who only serve political of financial purposes.