According to the beeb the police followed him from a house they had been watching and when confronted he legged it. I'm not at all happy about the police being trigger happy but if that one copper really believed that the guy was a danger to those people on the tube, and he had the power to do something, what was he to do? What would you have done in the time available.
It's pretty obvious the police would have preferred him alive for the information he held.
And as for the akenanoos of this world, do yourself. and us, a favour: nip over to amazon (do it through mudcat of course) and order a book called the Lion and The Unicorn, the author's name is Blair, oh sorry you may know him better by his pen-name George Orwell.