Per reports, 81-year-old William Brock told authorities in Clark County that he shot 61-year-old Loletha Hall outside his home on March 25 ... but defended himself, saying he only did so because he believed she was working with someone who'd demanded a ransom from him.

Long story short, Brock had been getting contacted of late by scammers -- who started out by telling him he had to cough up thousands of dollars because some relative was in jail ... which then turned into a straight-up ransom/hostage call, which left Brock confused/rattled.

At the same time, this poor woman was also being contacted by scammers -- either the same one or an accomplice, cops say -- who were pulling a fast one on her as well ... ordering her to go to Brock's home and pick up money they said he'd have for her.

Hall showed up -- also confused and not knowing what was going on -- and that's when this deadly confrontation happened ... with Brock confronting her with a pistol in his hand.

She had dash cam video rolling from her car ... and you can see Brock leading around the other end, with Hall screaming out for help. Cops say she tried entering her vehicle and fleeing ... but that Brock shot her and killed her, which you can hear in this clip, off-camera.

Cops arrested Brock and he's now facing murder and kidnapping charges -- with the police saying he had no reason to shoot this woman ... noting she posed no immediate threat to him, despite what was going on behind the scenes and in the lead-up to them facing off.

Uber says they've banned the account that ordered Hall -- who was literally just following orders that had come to her from the app -- from their platform. No word on whether they've ID'd, tracked down or arrested these scammers who initiated this awful tragedy.