Realistically, what can be done? Full police investigation of anybody known to have a gun?
The only thing I can think of is if someone knew he was a felon in possession of a firearm they could have reported it. But chances are he would have been let loose after a brief sentence and this would have happened anyway.
The lesson learned (for the 1,000,000,000th time in history) is that there are bad people in the world, and bad things are done to good people by bad people. There is no way to preempt their actions entirely. So we should be prepared to defend ourselves and our loved ones at all times.
Owning that gun didn't drive this kid off over the edge. It was apparent that he was dangerous based on the details that are emerging about him now.
But of course this will be exploited to the anti gun crowd's campaign for blaming the gun instead of the murderer.
“Realistically, what can be done?”
The only thing that can be done is for more people to love Jesus and fear God.