There are two reasons for laws. One is to provide a framework to punish those who who break them. The second is to act as a deterrent for those already inclined to obey laws (either due to obedience to authority or fear of punishment). Gun control advocates claim to want to prevent murders.
Murder is ALREADY ILLEGAL (and morally reprehensible) and already has a framework for punishment in place. Murderers are already, by definition, not inclined to obey laws. Therefore, gun control laws can only serve two purposes. To provide a framework to punish gun ownership (not murder, gun ownership) and/or to disarm those already inclined to obey the law (by definition, NOT murderers).