To safely employ a tazer in this situation, the cop would have needed body armor completely covering his head, neck, torso, arms, groin, and legs. Wearing anything less than full riot gear, that attack posed an imminent threat of death or grievous bodily harm. Striking the officer’s head or neck with a bladed weapon could destroy an eye, sever the carotid artery, or cause a wide variety of maiming or permanently disfiguring injuries.
Employment of a pain compliance method is only feasible once that threat has been stopped, delayed, or mitigated.
Neither of the officers present appeared to have had any opportunity to use a tazer or less-lethal device to stop the attack.