I'm not yet convinced that he was truly hearing "demonic voices" telling him to slaughter his former classmates. This "devil-made-me-do-it" claim may be a cunning ploy on his part in an attempt to mitigate responsibility and garner sympathy - and it seems that his claim has been somewhat successful in terms of creating doubt here on this board regarding his culpability. *Buyer beware* of the possible snake-oil salesman!
According to reports, it appears that he was planning his massacre as long ago as two years. IMO, if "the devil", aka psychosis, is the culprit, he would have committed his atrocious act two years ago, shortly after he declared to whomever at the Dollar Tree that he was going to shoot up the school.
Question: Was his declaration two years ago accompanied by any other symptoms or observable behaviors such as: self-reported delusions or hallucinations, observable incoherent or incomprehensible speech? If not, it's more than likely that he was completely lucid when he made his declaration and was not under the influence of a psychotic episode, IMO.
IME, psychosis-induced actions tend to occur imminently, while under the influence of an acute or transient psychotic episode - not two years later.
The accounts reported by his former classmates (following the break-up with his girlfriend) of his violent death-threats toward his ex-girlfriend & others tell me that his murderous actions were meticulously preplanned. IMO, he was seeking revenge for a collection of perceived wrongs that he had been cataloguing.
Until & unless more convincing evidence is provided regarding his mental state, I'm not currently persuaded that his heinous rampage was due to any alleged or bona fide mental illness.