If you have a mentally unstable person living with you, and you leave a gun on the kitchen table and go to work, only to come home and realize this person went out and killed 25 people, you can't just shrug your shoulders and say ... " well it's not the gun's fault, it's the mental disease". Of course it's the mental disease, but you made it incredible easy for this to happen and that's part of question.
Yes. Of course, with every right comes a responsibility, and in a scenario like that, whomever that gun belonged to would incur massive (criminal, even?) liability for failing to secure their weapon.
No one is suggesting that we start passing out weapons to the Trick-or-Treaters.