They've released the police affidavit for SM. Here is the most comprehensive story I could find. Lots of details and potential answers to questions on this thread:
http://www.fosters.com/apps/pbcs.dll/article?AID=/20130506/GJNEWS_01/130509438
I can't find a link anywhere to the original documents-- does anyone know where to find it? (Usually they release these in pdf form along with the news story, correct? They released the indictment docs, for instance.)
So it really sounds to me like KM was centrally involved in the murder. I also suspect that the Rochester couple made some sort of deal with prosecutors (since it sounds like they were aware/involved to some extent, possibly criminally liable themselves), because of that detail about the conversation with KM being recorded by "Roberta" right before xmas-- and then a couple days later they arrested KM. They probably let the Rochester couple off the hook if they were willing to "wear a wire" so the prosecution could get KM. I'm thinking any future trials may come down to how much she was "under the influence" of SM based on her age and vulnerability being the female and "sub" in the relationship-- it just doesn't seem to be in question, IMO, that she was involved in at least the coverup and disposal of the body, if not the act itself.
Also, I wanted to share an odd experience I had. Synchronously and just by chance, I was face to face with KM at a local business here in NH just the day before they released the police affidavit. It was surreal and quite creepy because she seems to be an utterly normal young woman, polite etc. just working a typical retail job. I recognized her but obviously did not say anything (she rang us up).
I also went to high school with SM, and to UNH for college, so this whole thing has hit very close to home (my high school was very small and I had many classes with SM). He was definitely "different" and quite socially awkward-- very much into fantasy books and martial arts (he'd often do "demonstrations" for the whole school at assemblies)-- but other than that I found him to be rather intelligent and generally pretty nice. He did have a creepy vibe to him: it was a running joke of mine in fact that I was always nice to him because I didn't want to "end up on a kill list." Not that funny now considering the events, but nevertheless it's true that you could tell something "sinister" was going on with him.
Now that I'm thinking about it, I do seem to remember him making comments from time to time that seemed to suggest a) a break with reality, or rather an absorption into fantasy that was borderline delusional (I'm talking things like him speaking from the point of view of a fantasy character in otherwise normal conversation, and pretending that dragons or other fantastical beings/animals were real without really acknowledging he was joking-- i.e. he never "broke character") and b) a sort of "coldness" where he would talk about killing "ninjas" or "enemies" and various graphic ways in which you could do this using swords and such (for example in his martial arts demonstrations he would say things like "See what I did just there, that took out 4 enemies, decapitated one etc. etc." even though obviously there was nobody there... and usually he followed his comments with a peculiar inappropriate laughter). It was odd but I think most of us just found it kind of funny and maybe felt bad for him because he was socially inept with much of this stuff and didn't seem to perceive that others found it absurd. Also, I know it's not really a rational thought, but I can't help but wonder if we, as his classmates, could've done something different to make him feel more included or "normal" or something (not that he was actively outcast or bullied or anything, but it was a very small community and it was probably very easy to feel alienated). And who knows if it would've made any difference anyway, and it's obviously no excuse for him, but still it gives me pause for thought about the nature of socialization/fantasy/alienation.
Anyway, as for KM, I am actually surprised she was let out on bail considering the possibility that she was actively engaged in the murder and to such an extent-- and in the Fosters story it says she and Mazzaglia had sex the next morning because she was "excited" by the murder the night before (as well as the detail that she "finished the job" in terms of LM's body not sinking initially). Seems so creepily inhuman to me.
and thank you very much for your very interesting and insightful first post! 
