Before the bodies cooled Tuesday after a deadly terror attack in Manhattan, conservative commentators raced to proclaim that a good guy with a gun might have stopped the speeding truck that killed eight on a bike path along the Hudson River. This is absurd. As in Las Vegas one month ago, no good guy carrying a gun would have made a difference in New York on Tuesday. A casual bystander with a pistol would face near-impossible odds in trying to stop a speeding truck.