It may not make sense, but it happened. People become cops for many reasons. Some might be insecurity and a need to control other people simply so they can feel better about themselves. Add adrenalin and poor training, and you have a public execution.
What, exactly, did happen? Was it murder? Was it an accident? Was it justifiable homicide? Was poor training involved? Facts are immutable things! I wasn’t there! So, at the moment, I’m not in possession of the facts!