I can see it from both viewpoints and I think the Police were damned if they did and damned if they didn't.
I saw on last night's news about the shooting marking a new policy - it's the same as the Israeli Army use with suicide bombers. If someone is a suicide bomber they can detonate the bomb in less than a second. A warning shot is no use: they can still detonate the bomb. A shot to injure is no use: in the torso is likely to detonate the bomb, a shot to anywhere else but the head still gives them time to detonate the bomb. This is why they shoot in the head - to kill. Five shots because apparently people have survived with three.
If any of the possible scenarios people have described are true: he was deaf; he didn't speak English; he didn't know they were Police; he had some other reason to run (illegal immigrant, drugs, whatever) then yes it is absolutely tragic.
But the Police there had to make a split second decision. He came out of a house which was under surveillance after the bombings, he could have been another suicide bomber, he didn't stop when warned, he went on to a tube train. They had to make that decision there and then. I couldn't do it. I can't imagine how they must feel. But if he had had a bomb and they had hesitated - and he had detonated it and killed x number of people - we would be baying for blood at how Police had watched a suspected suicide bomber board a train and done nothing.
They had to make an awful, split second decision and they made it. None of us would want to be in that position, would we? It's terribly, terribly sad and I feel for his family and whoever pulled the trigger.