He had no idea the lad has been taser'd, the woman police officer took a massive gamble with her shot from the back of the room and it paid off, I doubt she had time to shout " taser taser taser" which is standard procedure so he would have not know it had been deployed.
When the lad fell from the taser, the officer had his back turned after managing to get him off him after a relentless attack to the back of his head. He turned and the attacker was on the floor, He had is own taser pulled and both hands were occupied, he had no clue what could happen in the next few moments so he used his only available free limbs.
You all have to understand these officers are trained to "Shoot" potentially kill if the necessary arose, If the officer in question didn't have his taser in hand his next move would be for the glock.
Theres few times where an example can be made of thugs / hooligans and these videos should be used as a tool for the police to show and say " **** about and find out" we should not tolerate anyone assaulting a police officer like this, or anyone for that matter. And if you do this it is the first form of punishment you will receive as well as lengthy jail sentence.
Policing in this country has gone soft, from just stop oil being allowed to block major highways up to out and out looting during riots. There's too many communities and gangs roaming around with zero respect and zero fear of consequence for their actions.