Well, what would you do - guy you think is a potential suidice bomber, fails to stop when challenged, runs away into a crowded tube carriage (where 6 people have in the last 2 weeks attempted/managed to detonate devices) and you have at best a few seconds to make a decision? And the guidance/policy is to shoot him in the head if you believe he is a bomber?
Like I said a heck of a call for anyone to make; even if the guy turns out to not be carrying anything, I would not like to second guess the three cops in the situation. Though I agree, police officers tackling and shooting a suspect whilst he is on the ground is grisly and disturbing.
Could a QRA pilot find himself in a similar (though even worse) position someday?