"20/20" continues with unfriended. Here again, Ryan smith. Reporter: She's a killer nobody would have suspected, the fresh-faced high school senior, Shelia eddy, smirking in her first court... See More
"20/20" continues with unfriended. Here again, Ryan smith. Reporter: She's a killer nobody would have suspected, the fresh-faced high school senior, Shelia eddy, smirking in her first court appearance. After months of secrecy, the truth is revealed. Skylar neese's best friends Rachel and Shelia were actually her worst enemies. The shocking news they had lured her into the woods and brutally stabbed her to death. The search for Skylar neese is over. Rachel confessed. Two teenagers implicated. Reporter: Many of Skylar's friends hear about it at school. It devastated me. I actually had to get up and leave class. If you don't want to be friends with anyone anymore, you delete them on Facebook, like, you don't go and get rid of them. Reporter: Kelly Kerns has known one of the killers, Rachel, since the day she was born, considering her like a daughter. When someone tried to tell me that they stabbed them -- I -- no way in heck. Reporter: You couldn't believe that was your Rachel. You can't tell me these girls stabbed her. And it wasn't until I opened the paper and saw guilty, and her picture. Reporter: Today, looking at the photograph of Rachel taken just hours after she murdered her friend. Kelly is floored by that smiling face. What strikes me about this picture is, I don't see anything on her face that reflects what she did. It just makes you feel, you know, what did you miss? Reporter: And at the heart of the wrenching questions, how could two teenagers kill their best friend? They say they just didn't like her anymore. Which is typical of teens. But teens typically don't kill each other when they fall out of like. I think it was because Skylar knew stuff about them that they didn't want out. Reporter: Stuff he says was hidden in Skylar's diary where he says she wrote about a sexual relationship between Shelia and Rachel which she witnessed at a sleepover at rhel's house. They describe the incident in intimate detail in the new book they've written called "Pretty little killers." Shelia and Rachel became involved sexually, it appears. Skylar was forced to watch, because she knew if she left the bedroom they had all been drinking and they'd all be in trouble. Reporter: Several weeks after the sleepover, Skylar tweets "I'd tell the whole school all the I have on everyone, which is a lot. #Ificouldgetawaywithit." Then one month before she is murdered there is another tweet from Skylar, "Just know I know," Rachel and Sheila had formed some type of a sex relationship that they believed Skylar knew about it and would ultimately tell everyone. Reporter: But the neeses say that Skylar never would have acted on the veiled threats. She had a lot of gay and lesbian friends. It didn't bother her. You know, she didn't care if you were pink, purple, yellow. That was just Skylar. Reporter: Why do you think they killed her? For the thrill of it. To see if they could get away with it. They had a strong dislike for Skylar. So it's fitting they stabbed her in the back. Reporter: A year and a half after the murder, Rachel faces justice. She pleads guilty to second degree murder, a lesser charge based on her cooperation. Finally, in court, she directly addresses the neese family. I am so sorry. I don't know if there is a proper way to make this apology because there are not even words to describe the guilt and remorse that I feel each day for what I've done. The person that did that was not the real me. Not the person I am, not what I'm made of and not what I believe in. Reporter: Rachel's mom weeps as she listens. And I'm so sorry and I pray each day for everyone involved and I pray each day for forgiveness. She can take her apologies and everything else and sit on them because that's about what they're worth to me and my wife. She has done nothing but make our lives a living hell since day one. Reporter: She was crying, she was emotional. You didn't think that was remorse? To me, that was her final act in front of an audience. She's the big actress, the star of the high school plays. This was her last Broadway performance. Reporter: At her arraignment, a defiant Shelia eddy pleads not guilty. But later, in a surprise development, changes her plea. How do you plead? Guilty. Reporter: Guilty to first degree murder. She offered no words of remorse. Why do you think that Shelia has never apologized? Because she's not sorry. You don't apologize for murdering somebody in cold blood. Because she meant to do it. It's a good thing she didn't give an apology. People think it's horrible because she didn't. I commend her. Don't stand up there and lie to me and tell me you feel guilty. Reporter: Rachel's family described her actions in a statement as "Unforgivable and inexcusable." She is now at a juvenile facility, sentenced to 30 years in prison. She will soon be transferred to adult prison. The same prison, in fact, where the Sheila, the girl she turned in, is now serving a life sentence. But both girls could be out on parole by their early 30s. If I'm dead and gone, somebody will be there to make sure they talk to the parole board and these animals don't get out of their cages. Reporter: If you could say anything to Shelia and Rachel right now, what would you say? Rot in hell. Reporter: Skylar's friends are still trying to process their loss and mourn the milestones they are experiencing without her. Skylar never got to go to a prom. Mary and Dave never got to see her get all dressed up, and get the bouquet, and get everything else. Reporter: In memory of their only child, Dave and Mary neese helped pass Skylar's law in west Virginia. It requires amber alerts for all missing children. This is Skylar's room. Reporter: When you come in here, does it give you any sense of Skylar now? Yeah, it still does. Reporter: They continue to treasure every item that belonged to Skylar. Some of them don't mean anything, but they were hers, so we'll always keep them. Reporter: She has a bunch of cards and drawings. Yeah, see her friends were always sending her "A birthday wish for my friend." Uh-oh. That's what I think of that one. Reporter: Yeah. That was from Shelia, by the way. Reporter: The card reads "Happy birthday, love Shelia. Bff. Love you like a sister for life." For Dave, it's just one more reminder of a searing betrayal that has destroyed their world. Because of that outward appearance, people don't want to believe they're capable. I've still, to this day, have people say there's no way it's just those two girls. Yes. It's those two girls. This crime to me is one of those crimes that go down in the books as truly evil, best friends from hell. There couldn't be a more fitting description. Reporter: Today on a hidden road, deep in the woods of a town called brave, there is a silent shrine. The site of Skylar's murder has been transformed into a memorial. On this visit, a stranger had written a note to Skylar. This one says, rest in peace, sweet Skylar. Soar with the angels. Baby, I'm so sorry. If there's one lesson to be learned in here, and one lesson only, know who your friends are. We just heard that with good behavior, they could be out by their 30s. The question tonight, way
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