The unemotional answer is that the murderer was a convicted felon, not allowed by law to own or even possess a firearm of any kind. By the very definition of who they are criminals do NOT obey the law. Violent criminals are concerned about obtaining and using the tools that they feel they need to do what they want to do.
Disarming those of us who obey the law, and are a threat only to criminals will only make life easier for violent criminals. Witness England where self defense, armed or otherwise, is a crime in itself and the criminals run amok. Of course this sort of logic, the use of facts and data, has no effect on the gun banners.
Fifteen or twenty years ago I watched a program on PBS in Seattle where one of their reporters interviewed several men who were guests of the state for murdering someone. It turned out that none of them had shot anyone, but they all said that if they wanted a gun or whatever other weapon to kill someone it would be no problem to get one on the street. When the reporter asked what sort of gun laws could be passed to prevent "gun crimes" they looked at him like he had obviously not listened to anything they'd said. He hadn't either.
The gun banners use their "logic" which tells them that if we just restricted gun ownership or possession to politicians, police, and the military we'd all be safe. They don't want to be bothered by facts. They're also a bunch of sissies who are afraid of guns, defending themselves, or any thoughts that might involve anything to do with self defense.
Sorry about that, off my soapbox.