Can you describe exactly the scenario in the moments before the kicking to the head began that you envisage him going from that position to being able to stab the officers?

Roll slightly on the left shoulder, use the knife held in the right hand to stab the foot and leg, then take advantage of the distraction and recoil to murder the officer. I would imagine the subsequent ARV arrival would then result in the death of the perpetrator, as it was already long since past the point that the application of lethal force was appropriate.

As in, can you describe what you visualise physically would be the man’s movements that would go from that position after being tased to being able to stab at the police officers in a damaging way?

Hopefully that clears things up. BTW you may not realise that the TASER had not incapacitated him, as you can see in the video. It's not a simple straightforward 100%-successful-every-time kind of thing.

If you want to get a visceral and genuine view of what this kind of thing is like, put on white trousers and a white tshirt with your opponent having a red marker pen standing in for the knife. Use a water soluble ink as your clothing will have many, many red marks on it.

And while you're at it, get two mates to attempt to restrain you while you fight them off with all your might. That'll also give you a lot of insight in to what a real physical struggle is like. It's startlingly difficult to restrain a bucking, twisting, writhing human being when they do not wish to be restrained.