LEGALLY possessing a gun is a victimless crime. ILLEGALLY possessing a gun is not. This man was ILLEGALLY possessing a gun.
Who was harmed by him merely possessing the gun? The harm was in shooting the gun but he wasn’t convicted of that. It’s not much different from if he committed murder while he was jaywalking and they acquitted him of murder but got him for jaywalking. What he was convicted of had nothing to do with Kate Steinle. No justice was meted out for that crime.