I agree with you.
And I think focusing on the who is the wrong focus as well. The correct focus should be on the where.
I you look at the where of the mass murders occur you can see the they are mostly is designated Gun Free Zones. The Mass Murderer when planning his act chooses a place where he assured of little or no resistance to his murder spree. It is a logical choice.
But in reality mass murders account for typically less than 100 of the nations 10,000 or more homicides committed each year with a firearm. Again looking at the where we can again locate the place where the bulk of these crimes occur. We find that these murders are typically concentrated in major urban centers like New York, Los Angeles, Detroit, Washington D.C., Chicago, ect.
One thing these cities have in common is restrictive gun laws. Certainly these gun laws are ineffective as indicated by the numerous homicides committed with firearms.
Having decided where to look for the problem with individual firearm homicides we may decide to look at the who. When we look at the who in the case of the urban center homicides we by in large find a urban gang member involved drug related activities.
So if we wish to reduce gun related homicides perhaps we should focus on reducing the gang activities in our urban centers. As the author says in the article persons wishing to commit a murder will find a way to achieve that end.