i think the police did what they felt was right at that moment in time. They had to act fast in view of the terrorist threat. He fitted the description, he ran, which gave them even more reason to believe that he was the person they were looking for. They shot him before he had the chance to detonate a bomb, which they believed at the time was his reason for boarding the tube. Had they hesitated and he had been a suicide bomber, he would have just pushed the button and blown everyone to pieces. In that sort of situation, the police have to act fast and in the publics best interests, and this is what i think they did. Turns out they got the wrong guy, but things would have been so much different if he had been a terrorist and and they had not acted. Mistakes happen, i dont see why anyone should resign over this. They did the right thing.