My thoughts is a man who went wacko in his old age. I’ve known several who started showing signs of mental aberrations as they got older. Most committed suicide. One even hanged himself in the same barn his dad hanged himself. Same rope.
The taking of so many guns he would never get to use, into the room, and meticulous planing is to me the give away, much like the Aurora Colorado theater shooter.
Time to reopen the mental institutions for such people.