OK, then maybe don't jump to conclusions. One possibility is that it's a training video for a "watch this then report what happened" sort of thing, which are very common and may not even be affiliated with an actual police department.
Consider the presence of a camera crew (the filmer is way too close to an armed officer to be a bystander). Also notice how there is no blood and no physicality related to a body being hit by bullets.
For all we know this could be a training video to help cops avoid this sort of thing. What sort of twisted world do you live in where you would imagine that any police department in America could explicitly set out to train its police to cold-bloodedly murder people?
Everything related to this video screams of it not depicting an actual event.
Edit: videos of the police committing murder or negligent homicide do exist but there is generally far more context with those videos and they are tied to well known cases.
Edit2: I've watched the video a few more times now and I've noticed some important details which aren't very obvious due to the low resolution of the video. First, the civilian has another gun which he retrieves from behind his back, you can see it in his hands in the last few frames of the film. Second, the "police" are not in uniform nor do they have any police insignia present yet they are wearing bulletproof vests. If they were plain clothes cops they would not have their vests on above their clothes. If they were police on duty they would have badges, insignia, etc.
My guess is that this is a training video or video of training, maybe for police, possibly for private security.
