He wasn't a gun nut. He was a madman who, if his conditions had been properly reported, would never have committed his crime under current law. The only way to modify gun laws to prevent crimes like his is to prevent people with clean records from owning guns. Which amounts to a gun ban. Anything else is a feel good measure that does nothing more than restrict the innocent.
Existing law would have also prevented this.
The sheriff has stated that he has previously publicly threatened to kill people. That is a felony and if the sheriff had done his job he wouldn't be able to legally own or purchase a firearm.
