A majority of the injuries and deaths from guns come from indiviuals who are not licensed to own a gun. These individuals admit obtaining these firearms from someone else.
If the firearm is lisenced to an individual who did not keep the gun locked up; then that person should also be held responisble for the consequences of someone taking their gun.
Depending on what those consequences are they could pay a fine or even go to jail. He should also lose the right to own and buy guns.
The gun owner was negligent in keeping his gun out of the other individuals hands, which means he was an irresponisble gun owner.