Murder requires intent to kill. Prove that this cop intended to kill the "victim".
Intent to kill can (and often is) proven through the actions of the defendant. Here, the defendant--a police officer trained in the use of firearms and the laws governing the appropriate use of deadly force--unholstered his firearm, pointed it in the direction of the victim's head, and discharged it from a distance of approximately one foot. The prosecution will have no problem proving intent.