It has nothing to do with any decline of the nuclear family.
The Columbine killers were both from "normal", well to do, nuclear families. As a matter of fact, one them had a mother who was very active in one of the local chapters of the Million Mom March which was championing more gun control. Maybe she should have championed more "kid" control. One of them had a lock on his bedroom door and there was the sawed off portion of a barrel from a shotgun on his dresser which was found during the investigation. If one of my kids had a paddle lock installed on his bedroom door, that lock would be sawed off post haste and the door would have been removed from then on. We aren't always our kids "friends". Sometimes, we have to be their parents.