What people seem to be overlooking is that Keith was a psycho and total control freak. You could see that in the way he controlled everyone, even before the initial incident in the barn. Then later, with the gun, no mentally stable person behaves like that. That behaviour showed just how mentally UNstable Keith was. Keith's wife was cowed, as were all the other children, apart from Molly. They must have lived their lives in constant fear that Keith wouldn't suddenly become enraged over some trivial thing. The clue is Ryder's mom near the beginning of the movie not wanting his gayness to be mentioned once they got to the farm — and Keith. She well knew what kind of person Keith was. Everyone walks around him as if on egg shells.
What's worrying is that the world is full of Keiths who haven't been discovered yet. The spate of campus killings in recent years is just the tip of the iceberg. But, yeah, let anyone have a gun and they become fascinated in what it could do, what they could do. How powerful they feel when holding it. How much more powerful if they used it! And isn't it strange how so many of these psychos — Keith the character, as well as other real-life murderers like that guy in Norway and many in America — are nearly always neatly dressed and well-groomed — as if they're trying to hide their inner turmoil from the world.