My one major point of dissent is that I believe AW had a much bigger part in this, even if she did not do most of the stabbing.
Hi Ausgirl!
I think you may be right. MG has been shown to have serious mental health issues including psychotic episodes. AW has not received the same diagnosis,although her defense is contesting the doctor's report.
So... a believable motive for this attack is that AW was vying for PL's position as MG's best friend. According to a New York Times article, classmates said the friendship between the three girls was not equal (PL and MG were very close, AW not so much) so it may be that AW wasn't having a lot of success in winning MG over by "normal" means. From the various articles, it doesn't seem like AW and PL knew each other particularly well - only due to their mutual friend, MG.
According to ABC, PL's mother said that MG and PL had recently been getting into arguments and MG had become more controlling. This was upsetting to PL who just wanted to make things right. She didn't seem to understand what she had done to upset MG. MG persisted in sending PL Slender Man stories even though PL had asked he not to because they terrified her. PL believed that Slender Man was real, because MG had told her so. So a wedge was being driven between MG and PL. Was this change in MG's behavior in part due to deliberate actions by AW?
Noticeable by its absence is any reference to PL until MG allegedly suggested they kill her. I'd lay odds that there WERE prior conversations between MG and AW about PL. I can imagine them discussing PL's negative views regarding Slender Man, and the question of PL's loyalty as a friend arising as a consequence. In AW's statement, however, the first mention of PL is when MG "surprises" her by suggesting they kill her... but AW does not show any curiosity as to why it should be PL and not someone else.
AW could have been "feeding" ideas to MG. For example, the idea that the girls become proxies of Slender Man originates - according to AW's statement - from MG. AW makes it seem as though this was something that she had never considered, and did not know how to accomplish. However... the topic HAD come up before in connection with AW. Around January 3rd, on a Slender Man forum, someone asked how someone could become (by choice) a proxy of Slender Man. The questioner created a new account in order to post, employing the same user name as a Tumblr account owned by AW's sister. Internet searches for this name only show up in connection with these two accounts. The uniqueness of the name, the subject matter and timing make it hard to believe this is a coincidence.
Assuming AW is found competent to stand trial (there's a review of this in December) I imagine the prosecution will focus on AW's intentions and her understanding of MG's state. I think AW was aware that MG was undergoing mental issues - she even refers to MG as having a "mental breakdown" in the bathroom.
Another factor that we don't know is the true nature of MG and PL's "best friend" statusbefore the arrival of AW. Was this equally reciprocated, or was MG not, in fact, as close to PL as PL was to her? In the ABC article PL's mother states that PL was extremely excited about the sleepover. On the other hand, MG teased PL in comments made on PL's Google+ account regarding PL's love of Katy Perry's music. This was only 4 weeks prior to the attack. (The account was closed down around the time PL returned to school).
I imagine that it will require input from PL in order for us to understand the true story leading up to the attack... as well as forensic evidence from iPads, laptops, and phones.