This was an execution by a trigger happy cop. They had plenty of opportunity to handcuff the suspect. The cop was allowed to use his own personal AR-15.
If this cop was so in fear of his life he should have called in backup. He should have ordered the suspect to lie down on his belly will all arms and legs splayed out and not to move. There were other police on the scene. They all should have converged on this drunken suspect and taken care of arresting him.
Might have a lot to do with it if this cop liked to run around on his off time in the desert playing "operator" and was just itching to try his AR out on a live target.