Ah yes, the "If guns are outlawed, only outlaws will have guns" argument, with an interesting twist about bent coppers. Of course "unbent" rozzers have guns as well.
I don't know whether "mad people" with guns shooting up public places own their guns legally or not. AIUI, it doesn't seem to matter much. In a society where guns are owned legally there is no lack of ways to get them illegally.
regards,
Hein