It's insane, scary and believable ... a 19-year-old girl admits to not only killing a guy who hired her for sex on Craigslist, she says she killed so many others, "When I hit 22, I stopped counting."

Miranda Barbour is in a Pennsylvania jail, awaiting trial for allegedly stabbing Troy LaFerrara 20 times, because she tested him and said she was only 16 and he responded by saying he'd still have sex with her. And -- as a molestation victim herself -- that set her off.

Barbour said in a jailhouse interview over the weekend she's been killing people since she was 13. She insists she only kills bad people, claiming, "I did this to people who did bad things and didn't deserve to be here anymore."

In an interview with the Daily Item ... Barbour says the first murder coincides with joining a satanic cult in Alaska.

As for the LaFerrara murder, Barbour says she and her husband of 3 weeks killed him in their car.