I doubt that many people will agree with me, but I really don't see AS as a sociopath - a very sick minded person, but not a sociopath. There are a few things that just don't fit that label for me. For one thing, he doesn't seem to be all that charismatic, which is a common characteristic of one.
People have said he was intelligent, and friends thought he was charismatic, but it seems like most people described him as strange, quiet, weird, etc.
I don't believe he and his brother would be as close if he was truly a sociopath. It seems like little brothers and sisters are almost as commonly abused as pets by people with no conscience, but his brother seems to genuinely care about him and doesn't seem to have accepted it readily.
He doesn't seem to be much good at lying. I don't see a true sociopath confessing to his mother to start out, and immediately admitting to the murder and other things once questioned. Being a believable liar is a big trait of sociopaths, but it seems like almost nobody has believed much of anything he's said (other than that he's guilty!)
To me, he seems more like someone who is fairly high on the Autism spectrum, but has other severe issues along with it. The signs I see are: not knowing proper social cues, becoming obsessed with subjects of interest, appearing "odd" to people who don't know him well (and to some who do), not spending time doing age appropriate activities, spending time with younger kids (especially girls, because he seems to have more in common with them), the stare is fairly common in someone who is trying to force him/herself to look people in the eye, sometimes a vacant stare can actually aim at another person and feel uncomfortable.
My guess is that he has some form of PDD, along with anxiety, depression and/or some other mood disorder, OCD, and probably some form of psychosis. I believe he knew what he did was wrong, but I'm not sure how much he cared, I really wouldn't be surprised at all if he were to plead guilty. I'd like to say that I feel sorry for him, but any chance of that flew out the window the moment his fantasies became reality.