>With the Sandy Hook shooting, was the owner of the gun prosecuted for allowing the shooter to have it?
No, because she was the first person killed by the very weapon.
You don't see how, if people know they are legally responsible for what guns they own are used for if they give them to someone else, they would be more careful about who they give them to?
Obviously prosecuting someone after the fact is retroactive for that particular act. But that in no way means it does not send a signal to other people to proactively change their behavior. That is how most law enforcement has its effect. And it has the advantage of putting the responsibility where it belongs; whereas if you take away everyone's guns because a few people can't use them responsibly, or can't be bothered to be careful about who they give them to, you are punishing the wrong people.