Aaron Andrews himself seemed to believe the 9/11 WTC cleanup efforts had brought on PTSD, and that this PTSD was responsible for the voices in his head. Such auditory hallucinations are, sometimes, a symptom of PTSD.
But of the time, PTSD is characterized by 1) Reliving the experience, 2) Avoiding thoughts, situations, people, or places associated with the original trauma, and 3) "increased arousal", which means, heightened feelings of sadness, joy, anxiety, difficulty falling asleep, etc.
Schizophrenia and Bipolar are often characterized by auditory hallucinations, but the average age for the onset of Major Mental Illness, such as Schizophrenia and Bipolar, is usually in the early teens or early 20's.
Unfortunately, I have spent MORE than my share of time with drug addicts. Aaron Alexis does not look like a drug addict to me. While it is true that some people who are addicted to drugs are able to pass as normal, usually there are subtleties that someone like me can pick up on. He simply does not look like a coke or meth user. Heroin and other depressants would not cause auditory hallucinations. And there is no evidence that Aaron Andrews was using Hallucinogenics.
Aaron Andrews was 34 years old. And best as I can tell from the media reports, his auditory hallucination symptoms had only been presenting themselves for the last year or two, with a significant increase in the past few months. (It is true that he also exhibited anger management issues, and significantly so, as far back as 2004. But anger management is a problem for many people and ought not necessarily be conflated with the exhibition of symptoms such as voices in the head.)
This leads me to believe he may have been suffering from a brain tumor.
Hopefully, this possibility will be considered and checked into by medical authorities.