"We just didn't like her anymore"
This is the only motive Rachel provided for her and Shelia's crime. On the surface, it doesn't seem to make a lot of sense. That Shelia and Rachel were falling out with Skylar has been well established. But certainly there are better ways to end a relationship than murder. There were indications that the girls were not spending that much time together anymore anyway; while Shelia and Rachel were growing closer together, Skylar was seeking friends elsewhere and rekindling previous relationships. Not that she was very happy with this state of affairs; she'd often tweet about the good old times with Shelia and Rachel. But it seems that things were "naturally" growing to a point where the three would simply cease being friends anyway. So why murder her?
"They worried that Skylar would divulge their secrets"
As mentioned, Skylar witnessed Shelia and Rachel have sex on August 16, 2011; not only did she relay these events to some of her friends, but she also wrote about what happened in her diary. Furthermore, according to rumors, Skylar was in possession of videos and photographs of the two girls engaging in sexual acts. Therefore, it could be argued that the girls killed her to keep her quiet. However, there are several problems with this explanation. First of all, Skylar was a champion of homosexual rights and as such would never have given her friends a hard time for that, even if having to watch them have sex made her feel uncomfortable. (Who wouldn't feel uncomfortable watching their friends have sex, homosexual or not?) Skylar's friend Daniel, the same person who persistently kept bugging Rachel about what happened that night, was gay and had confined in her numerous times. Second, Shelia and Rachel often joked about being lesbians and Shelia had a reputation for being promiscuous - a reputation she seems to have enjoyed. It didn't seem like the whole thing would bother them at all. That it took almost ten months to go ahead with the murder indicates that there was no sense of urgency. (Of course they could have thought that their relationship with Skylar was still salvageable, at least until the big early-June 2012 fight between Skylar and Shelia.)
It is possible, however, that while Shelia probably wouldn't have been bothered by being outed, she still could have used this angle to manipulate Rachel, who was active in less tolerant religious circles and had many friends among them. Many believe Rachel is far more guilty than is generally believed, suggesting that it was her fear of being outed as a bisexual that drove the girls to murder Skylar. NewsBall claims that an earlier relationship Rachel had with another girl was broken because Patricia, Rachel's mother, didn't approve of it. Rachel also had an on-off relationship with Mikinzy Boggs that could have been endangered by rumors of a lesbian affair. In some of her tweets Rachel comes off as extremely jealous and when this is combined with her mental problems and sometimes violent outbursts, it is no surprise Rachel ended up doing what she did. But if she was the "master mind" of the crime, then Shelia has been curiously disinterested in dispelling the prevailing notion of her as the master mind.
The murder becomes less of a mystery if we consider the possibility that Shelia Eddy is a rather typical psychopath. The results of her psychiatric evaluations (conducted while waiting for trial) have not and will not be made public so, like above, we're left with speculations. Shelia inserted herself into the investigation right from the beginning and manipulated people (by faking emotions, for example) into providing her with information. Investigators described her behavior during interviews as "wrong"; arrogant, narcissistic and calm (while Rachel seemed scared and nervous). She arrogantly flaunted the investigators on Twitter ("no one on this earth can handle me and rachel if you think you can you're wrong", November 5, 2012) and tried to flirt with them. When she was arrested, she seemed worried mostly about how her (undeniably beautiful) hair looks, assuming there'd be people wanting to photograph her for the news (there weren't any).
Her behavior in court was even more revealing. She sobbed when it was about her, whenever hit (such as during her teary "Guilty" admission) with the realization that she is going to prison for a very long time. But when Dave Neese gave a tearful testimony, she looked absent, as if the whole thing didn't concern her, that, if anything, she had been in the right killing her "bestfriend". She pled guilty but did not assume responsibility for her crime; she only did what she had to in order to get over the proceedings as quickly as possible. Psychopaths consider themselves superior to society and its norms. If Shelia found Skylar annoying, then, in her mind, she was justified in killing her.
Many, particularly older people and former friends, have wondered and will keep wondering how a girl like Shelia could commit such a terrible crime; after all, she doesn't resemble a killer. And yet that's exactly what psychopaths are like: Unlike sociopaths, psychopaths appear normal to outsiders. People with dungeons in their basements and bodies buried in their backyards have been described by their neighbors as "nice and helpful". Psychopaths exhibit "superficial charm", making them appealing and chameleon-like - until you scratch below the surface and find there's nothing there. It is the sociopaths who act like they don't care (while they in fact do care at least about those closest to them). Psychopath act like they care but they don't. Their emotional life is shallow, but they recognize the value of expressing emotions in specific situations for personal gain (hence Shelia crying on Skylar's bed when she knew Skylar's mother would hear it). Their high They know certain things are wrong but they don't "feel" the norms. Unconditional love, the kind of love Skylar felt for many, is a completely alien concept to psychopaths. Their friends exist for them, to entertain them, to make them feel good about themselves and to gain power over others.
Casey Anthony killed her own daughter because she found her an inconvenience to her social life. Similarly Shelia Eddy killed her best friend because she didn't want to be friends with her anymore, because she found Skylar annoying and didn't like the idea of Skylar revealing their general secrets to her new friends. And she manipulated her weak, mentally unstable friend to help her do it. While Rachel might have genuinely felt at some point that they aren't really gonna go ahead with the murder (during her apology, Rachel said the person who committed the murder wasn't the "real me"), Shelia must have been looking forward to doing it. A common trait among psychopaths is that they get bored easily, which contributes to risk-seeking behavior. Murdering Skylar would have been what's known as a "thrill kill", the killing of someone for the excitement of the planning and killing. That'd have been Shelia's moment of gratification.
Early videos and photographs of Skylar and Shelia together are a bit hard to look at now knowing how it all ended up for them. They looked so good together, until they met Rachel. While some have, with the gift of hindsight, mentioned individual instances in Shelia's early life that seem to back up the idea that she always was a psychopath, age is relevant: Psychopathy is a disorder that usually arises in a person's early teens. Skylar, and others around Shelia, would have been helplessly unaware of this. Skylar's parents would have had no idea that Shelia, their daughter's "bestie" whom they loved as if she was their own, was growing increasingly indifferent to the feelings of others.
Of course, none of this alone explains the murder. Psychopaths can kill people and not feel bad about it, but that doesn't mean they necessarily want to kill people; psychopathy doesn't imply sadism. Psychopaths are also slightly above average in IQ: Even if they had a motive to kill someone, they might still consider it not worth the risks. Many, if not the majority of homicides are stupid in their execution: A guy walks into a liquor store, gets a few hundred dollars from the register and then shoots the cashier in the face. Even some supposedly thought-out murders appear downright farcical, as if from a Coen movie. Murdering Skylar might have been the most stupid thing the girls ever did, but it still took six months to find out what happened that night, and even then only because the weak link confessed. For sure, the investigators and friends of Skylar helped by applying pressure, but as far as evidence is concerned, they had nothing against the girls until Rachel came forward. The bloody clothes and murder weapons were never found. If they don't have a body, what are they gonna charge the girls with? Shelia, a big fan of Law & Order: Special Victims Unit (at least when Christopher Meloni, her favorite "DILF", was still in the cast), also made sure that all communication is done either on FaceTime or in person. The kind of fearlessness usually associated with psychopathy combined with normal youthful arrogance must have only helped her believe she could get away with it.
None of this is meant to imply that Rachel Shoaf is somehow less guilty. She committed the exact same crime as Shelia did. If she didn't really want to do it, she had plenty of time to call it off. And if she is capable of empathy, then that makes her all the more guilty. At least Shelia can always use her lack of empathy as an excuse. But Rachel should have known better.
Neither is Shelia any less guilty because of her supposed psychopathy. Psychopathy is not a mental disability that'd prevent a person from understanding the consequences of her actions, or from controlling those actions. If they had found a mental disability, the defense would have used it in court. But they didn't. Psychopathy is largely a result of genetics and cannot be cured. Thus Shelia will never feel any remorse for what she did to Skylar Neese, and will die (hopefully behind bars) blaming others for her imprisonment.
"Kids/parents these days..."
Usually when something like this happens, many instinctly take it as a sign that something's wrong with the modern world or the current generation of kids (and their parents). Individuals with such opinions comment with a sense of moral superiority along the lines of "If they had been my children..." or "My parents never would have allowed me to..." The reasons vary from "too much parenting" (Rachel) to "too little parenting" (Shelia). Some have even blamed the Neeses for failing to prevent Skylar from sneaking out in the middle of the night (should they have installed bars on her window?) or for allowing her to hang out with the wrong crowd. Of course, the "wrong crowd" in this case consisted of pretty typical teenagers with all the harmless pot-smoking and partying that go with the territory. All three were academically at least somewhat gifted (particularly Skylar) honors students. Even Shelia took college-level courses and had finished all her high school studies by the time she was convicted to life in prison. (She had been in custody since the end of her junior year.) There weren't any red flags.
As for what this tells us about the modern world; not much. In fact, crime rates across the Western world are at historically low levels. Homicides in the United States peaked in the early 1990s and are now at a lower level than ever in the country's history, except for the decade from the mid 1950s to mid 1960s.1 So, unless you were born then, you cannot claim things were better (in terms of crime) in your childhood. By many measures today's teenagers, and humans in general, are healthier, more responsible and more intelligent than ever before in human history. This should be kept in mind before trying to draw far-reaching conclusions based on what two deranged individuals did.
1 Homicide trends in the United States, Bureau of Justice Statistics, 2010.
2 Claude Fischer