My only thought about it is that it seems like blind fearful panic on the parts of the two LEO shooters. One shot from a perp and they massively and methodically unload their complete ammo cache down range willy nilly.....very professional.
This guy had just shot his mother and his sister in the head, and you think the police opening up on him is an overreaction? I respectfully disagree.
My cousin retired as a Lieutenant of Detectives from the NYPD, working on the Manhattan DA's homocide squad to finish his career. He never drew his revolver in 20 years on the force. He actually once talked a gentleman distraught over a fight with his wife to surrender his weapon without drawing his own. If someone had ever taken a shot at him, I doubt that he would have hesitated to return fire with every thing he had.
Sounds like defense to me, not panic.