Adam Lanza should have been subject to the background check
because he was part of the household that the guns were taken to.
He had free access to them one time too many and that's all it takes
to end the lives of his mother, school kids, their teachers and staff.
You wouldn't want to find a way to stop that?
If he is blocked from having guns in the house, he has no easy access 
to guns. Easy access for a mentally-challenged kid like that can make 
the difference between merely fantasizing and acting out.
Without the easy access he can't go on the rampage he eventually did.
Simple.