I can think of lots of "damn good reasons".If your life is anything like mine none of them are very likely to occur but here's one that comes quickly to mind....
Your 22 years old son,or grandson,has been acting strangely recently.Nobody knows it yet but he's in the early stages of schizophrenia,approaching the middle stages.In one of the delusional states he's been suffering (unknown to anyone) he believes that people are out to get him.He takes his younger sister hostage,with a knife to her throat,and starts talking gibberish.The cops have been called but before they arrive he says/does something that makes you fear that he's within seconds of cutting his sister's throat.You fire...instinctively.
Tragic? Of course.But might that qualify as "damn good"?
Remember,Marvin Gaye's father had to shoot and kill his son who,it was reported,had been riddled by booze and drugs for some time.
I thought we were talking about strangers firing a weapon on my property,not me or a family member.Try again please.