Again, just because someone has been tased, it does not mean they are incapacitated.
If a firearms officer is holding a firearm, while trying to subdue a violent attacker, they are within their right to shoot them. Especially if there is a risk to public safety. In this case the risk is a violent attacker gaining control of a firearm, instead the officer used a free limb to help neutralise the attacker. Perfectly acceptable use of force.
I think the police have a lot of problems, and a lot of bad coppers have slipped into the force, but this officer did nothing wrong.
I'm fully in favour of all our police being armed with lethal firearms at all times. As they are in most developed countries around the world.