There is an awful lot of misunderstanding on this thread. The police are (regularly) held to account and cannot use force that is not proportionate to the threat they face. Body worn footage has now meant that incidents can be picked over at length in the cold light of day (which is no bad thing).
We now have a society where police are often reticient to use their powers for fear of criticism later. There is a line between holding police to account and ensuring that the police do feel empowered to use their powers as needed to protect us all. That is why ZP's comments are ill-considered and harmful.
Here, shooting him dead would have been considered proportionate. He is lucky they were not armed.
They used their taser. He did not drop the knife. They had every right to use as much force as necessary to make him drop the knife, which includes kicks to the head.
I cannot believe it has been suggested that they shouldn't have sworn at him. Yes, the police are taught to swear in order to exert control if needed to. The fact this is even suggested suggests how much ignorance there is around lawful use of force.