The cop could easily have incapacitated him with a stun gun or other non lethal force.
I have been a prosecutor and defense attorney for the last seven and a half years and I will tell you a cop who have been one for eight years with eight separate agencies is nothing but trouble.
They are referred to by respectable LEOs as gypsy cops.
Also, I have seen far worse where the cops successful used non lethal tactics to stop the threat.
And a couple of times it was a good thing because the suspect had been slipped a drug without their knowledge and went totally out of character.
This result can’t be undone no matter what is learned in the aftermath.
Assuming of course that he had access to such a device.