The Doctor watched the Master going down and Jack getting up and an icy calm finally settled over him. Everything went very clear, as if time itself had slowed down.
He stepped swiftly and neatly over the chunks of broken floor and scooped up Jack's gun. Aimed it at the Master's left heart and coldly, deliberately fired.
Jack might have been in the way as he fired. He didn't know, and at this point, he didn't care. Abruptly, he was over his problem with getting Jack killed. He just wanted the Master dead.