I don't know, but I have the impression that many of these gun lovers simply wait for an opportunity to use their guns, even when it is against their own family members. A 'heated argument' like described in the article was enough in this case.
- - - Updated - - -
If she only had a knife, the husband would have had a much better chance to stop her. A small weak women can kill her entire family with a gun, but with a knife that is a much more unlikely.