The hoodlum pic at the beginning. It must have dawned on him the consequences of actually pulling the trigger on Captain America. Beyond Cap being able to dodge the bullet (and force feed him the gun for his troubles) should the hoodlum actually deliver a killing blow- every cape, uniform, Western spy, law agency would want 5 minutes in a locked room with him. Most of the active and retired armed forces would kill him, and the Red Skull, Zemo and the like would resurrect him and kill him again just for depriving them of the opportunity to kill Steve.