I'm not commenting on the current sad circumstances... but in cases where a genuine suicide bomber is confronted by the authorities, fatal attacks - headshots - are the only safe course of action. You can't ask them to surrender then hope they do. They've nothing to lose, usually, and in a city like London there are always too many innocents in harm's way. Wounding attacks, like gunshots to the leg, still leave the target capable of activating a bomb. Body shots are out of the question, because shooting bombs is a dumb thing to do.
Like I said, though, this is not intended as a comment on recent events.