You are all missing the point really. They had this man under surveillence from a house they had been staking, a house police believed to be connected to the bombings. They followed him & in the course of going from the house to the station he had changed clothes & appearence & acted suspiciously (dubious word I know).
They tried to stop him & ran into the train. You, as an officerr have to make a decision on the spot, as it happens, right then. Personally, I'd of shot aswell. It's easier to live with a mistake that cost one life than an indecision that cost 20+.
What would have happened if they had hesitated, not shot to kill & he then pressed a button? Every thought passes through the mind of an officer/soldier in a milisecond & the decision needs to be made right then.