The only way to stop a bad guy with a brainworm is a good guy with a brainworm 
He needed the erection to steady himself for the shot, not unlike a tripod. He lost it when one of the cops popped up over the roof like a sweaty gopher, and thus lost aiming stability.
Or an ethereal bisexual long-legged socialist to purr into his ear to remember to exhale before firing, and to use the edge of his finger to pull the trigger so that the barrel doesn't jerk away from the target at the last second. And also to aim at the largest fucking center of mass, for fuck's sake this isn't an FPS even if you live your life feeling like Elvis from Perfect Dark.