Do you have a reference for Ivan being a registered gun owner? (Not a anti gun lobby reference from the internet - they aren't reliable)
At least two of his guns were illegal under the existing legislation. One was a fully automatic weapon. The home made silencer was also illegal.
Ivan was well known to police and had spent time in jail. Under current legislation he wouldn't be a fit and proper person to hold a license.
You are implying that banning guns will stop bad things from happening which is simply not true.
The vast majority of serial killers exhibit strong predatory behavior AND don't use guns. They prefer something quiet which reduces the risk of being detected.
Ivan had a long history of violence and rape. Banning guns isn't going to stop someone like Ivan. He's not a normal person.
