The actual killing was the culmination of an operation that was pretty evidently botched. It has all the marks of a panic attempt to rectify something that had gone badly wrong.

A man who is believed to be a suicide bomber is allowed to get on a bus and travel through crowded streets for several miles. When he is at the entrance of a tube station he is alerted to the fact that he is being followed, in such a way that he is able to get down to the platform and on to a train full of people. He is puyrsued and forced to teh griound so that he is immobilised and then shot eight times.

The actual shooting isn't by any means the only question, or even the cetral one. The real question is, how could it possibly happen that these officers found themselves in a crowded tube train with someone they were convinced was about to explode a bomb, so that it must have seemed there was no alternative but to kill him? How could this happen in the light of the fact that a sizeable team of experienced and trained professionals had been in his presence ever since he had left his home quite a few miles away?

There appears to be a level of professional incompetence here which is frightening. To adapt the words of the Duke of Wellington referring to his army "I don't know if they frighten the enemy, but By God they frighten me..."
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But I don't know where people get the idea that killing a bomber is a reliable way of stopping the bomb exploding. It'd be the easiest thing in the world to rig a device so that the very act of killing the bomber would set the bomb off. You wouldn't need anything more complicated than something like a clothes peg held in the hand, wired up to a detonator in a Dead Man's Switch arrangement. If they haven't done that already I can't imagine it'll be long before they do. They may be mad, but they aren't stupid.