With his age being at 66, I think that if he didn't have a gun, he wouldn't have killed the kid.
Plus, I'm sure that many kids had been messin with him over his lawn for a very long time.
People just snap sometimes. But if he didn't have a gun, then snapping would have just been yelling at the kid.
Personally, I woulda planted remote control landmines. When it says 'No walking on the grass', it's for a valid reason.