Maybe the insanity plea IS the plea to save the boys life?
Think about it: here's a 20 year old kid with a history of being bullied and outcast. He has a terrible family life. Bad parents. A mother that's high all the time. Mentally Ill sister. He doesn't bathe, is obviously depressed, and spends his life immersed in online communities and videogames. He rarely leaves his room. His social skills are stunted, and we can deduce from online communication that most of his direct communication had a child like quality to it.
He became obsessed with an internet clique genre of people who glorify murder, occult, satanism, and drug use. People he looks up to routinely garnish weapons, wear make up and fake blood, and write/sing about how cool it is to murder. They also glorify drug use, violent sex, and domestic abuse.
The internet clique is highly structured with the people on top considered (and considered themsevles) as gods (ie, apostles, anyone?). Sam is a low man on a totem pole, and is actively trying to climb rank by joining them as a rapper. This scene becomes his life. He's seeking the approval he has never had before. Yet, he's stuck. His music isn't very well-received.
He finds an underaged girlfriend online. He spends all of his money to fly out to meet her. As he leaves, he's rejected by his parents as they tell him he's no longer welcome home. He gets off the plane to see her, and she rejects him. They spend hours in a car together where she's cold and ignores him. At the concert, she goes off with the people he's trying to impress while he stays off in a corner. The rejection builds on the ride home. She and her friend are bullying him, the thing he left his life behind in California rejected him. Etc.
Could that sway a judge or jury? Throw some expert testimony in there about Sam's mental state? That makes it a toss up between whether you believe the state's expert or the defense's. Seems like it could be a pretty long (expensive) and intense trial if they pull the insanity defense. Maybe this is what the defense is going to hang their hat on in an attempt to save his life.
Maybe they enter an insanity plea until the DA offers LWOP.
A few loose ends: we keep pointing to what Sam said to the taxi driver and reading things into it. We can do that, but if the prosecution were to put the Taxi driver on the stand and say "What did Sam say to you?" "He waited for her to go to sleep and he left" and then somehow tried to make the point that what Sam MEANT was he waited for her to go to sleep, then killed her, that would be immediately objected to as speculation. Which it is. And is barred from the Federal Rules of Evidence. If the judge allowed it to continue: personally, I would object until I was thrown in contempt. Also, it would be an appealable error.
Also, if Sam's attorney is competent, he wont take a "demonic possession" defense when the insanity defense is certainly better suited and less, well, ridiculous.