He wasn't merely an 'Aspie.' He had a full gamut of personality disorders, narcissistic/histrionic/anitsocial personality disorder at the very least, and was likely slipping into full on schizophrenia as well.
I would say narcissistic PD and antisocial PD (with a good chance of also fitting criteria for psychopathy, which is quite rare even among ASPD), can explain the full range of his behaviors. Delusions and other thought disorder symptoms commonly occur in severe PDs. Though as vander mentioned, it is impossible to say without a diagnostic interview. Even NPD-like symptoms, which seems like a lock based on how he talks, could very well be better explained by ASPD+thought disorder, or the other way around.
Regardless, there is a really strong genetic basis for this cluster of behaviors, though environment always plays a role. Environment doesn't always have unique influence, though. We shape our environment through our behavior, so the environment often ends up reinforcing genetic predispositions. A kid who has difficulty relating to others is more likely to be rejected socially, exacerbating those difficulties, etc.
Prediction always comes up in cases like this, but there is substantial evidence that clinical prediction is considerably worse than actuarial prediction. In other words, mechanical, group-based prediction methods, with enough adequate predictors included, are better than a clinician's judgment individually tailored to the person, even if they have substantial personal experience. We just can't predict violence like this with any kind of reliability.
I'm curious what therapy the kid had. It's surprising to me that he did this despite being in therapy since 8 years old. Therapy for externalizing behaviors (eg acting out) is very effective for children and adolescents, when using the proper technique, of course. However, many rich kids go to long "mental health retreats" designed for rich kids with behavior problems. These programs have been studied extensively and have proven to be iatrogenic, ie harmful, ie worse than no treatment. There is a lot of science in the mental health fields these days, but it is heeded less often than not. Most go for nice sounding treatments instead of those supported by the science. It's the same as seeing an alternative medicine "healer" or whatever to cure cancer, but it's often motivated by ignorance ("there is science behind psychosocial interventions??") rather than mistrust of the establishment.
