What matters is that any brain weak idiot is capable of firing a machine gun in a crowd, but certainly not to wield a knife causing the same damage. Nobody will stop murderers from murdering, nobody will stop muggers from mugging. But what we can stop is unnecessary mass shootings, in fact most could have been stopped because the guns were not supposed to be in the hands of those who had them at the time, either because they should have been precluded due to mental health reasons or because they had acquired them illegally.
Have as many guns as you want, but if one of them ends up in the hands of somebody who should not have had it, and you are at fault, you should pay the price for the damage caused. And for that it has to be known who the legal owner of a gun is. And that is why we need a registry.