Perhaps they've been a gun owner in the past. I'm as lefty as they come, and I have owned guns in the past, and plan on having a gun again. There are people who hunt, who live in areas with bear and other dangerous animals, and there are those who have worked in fields where firearms are something you have. I have worked armed security. I enjoy going to gun ranges (though, as a general rule, I no longer do that. Too many dipshit right wing stains on humanity hang out there). I lived in a very rural area until April, and we would often target shoot out in the back yard. Owning a gun in the US is so normalized that if OP is in the US, and anywhere outside of major urban centers (and even then...), it is very possibly a concern that if they are not in their right mind, just popping down the local gun store (and there is almost always at least one local gun store around) or pawn shop, and pick up a 9mm or a .38 special. If I ever got a dementia diagnosis, this would also be a major concern for me. I would give my power of attorney to whoever I trusted most, and make sure the state was alerted that I should not be trusted with a weapon again. I've never considered this aspect until now, but OP is exactly right. It's far too commonplace a thing not to worry that in an episode you won't just go do that in the States. Our gun culture is absolutely insane.
Also, before the accusations fly:
There have been no children in the home
The guns are in lock boxes
Ammunition is stored separately
I am not a "but the home invasions!" Nutjob