The reason Nancy Lanza had so many guns was that she was a “Doomsday Prepper”. I think that revelations that Nancy Lanza knew her son was worsening and believed that Doomsday could be prepared for more than counterbalances the assertion by friends that Nancy Lanza was a “responsible gun owner”.
“I am Adam Lanza’s Mother” at Gawker is a great personal accounting of being mom of a kid with increasingly unmanageable mental health issues, but in reality, I would guess it was submitted before it was widely known that Nancy Lanza believed Doomsday was around the corner and that it could be prepared for by heavily arming herself and her emotionally withdrawn son. That’s what she felt responsible for. Preparing her family to fight some looming war so they could survive the end of this world on this world.