This is the typical “spring gun” case - where you set up a gun to automatically shoot someone if they open a door, step on a pad, etc. It is HIGHLY illegal - because there is no human judgment involved, and no danger to human life. The landowner is in big trouble if someone wants to make it for him - notwithstanding that the device in question was not lethal, nor intended to be so. It could have caused a serious injury, and it could have resulted in this older man having a heart attack.
It is NOT a good idea to do this kind of thing. Don’t like trespassers? Fine, no one does. Set up cameras and then pass the footage on to the police or rangers. Be there and make a citizen’s arrest, and wait for the cops to show up. Or, in this case, simply talk to the person in question. This guy was pretty harmless, and had no ill will at all.
I think there was a case in Phoenix a decade or so ago where a guy’s small warehouse was repeatedly getting broken into. He set up a shotgun triggered by the door opening and, sure enough, the bad guy entered, and his head was filled with buckshot.
The building owner went to prison.
If you are going to do something like that you need to pre-prepare for a place to get rid of the body and evidence (blood).