I didn't watch the video. Videos very seldom tell the whole story. There is so much more to an investigation than a video, however, having video certainly helps to support someone's version of what happened and videos are great for identification purposes. It never tells the story of how we got to that point. If this cop killed without provocation of any type, then he should and will be prosecuted.
In every police department that I am aware of, there is a code of ethics. There is some truth to the stories that we have all been told that the police look out for each other. I would never deny that. However, when one of the Officers or Troopers cross that line, they are as susceptible to prosecution as any citizen, except maybe politicians and celebrities.