I agree that it’s most likely he just wanted to kill a lot of people, along with himself.
But, late-onset schizophrenia is a thing and it appears in men during their 40s. I’m sure there are plenty of other psychological issues that either only appear later in life, or that some people are able to mask/hide. My point was that it’s a possible explanation, not that it was the most likely one.
Edit: Looks like “acute psychiatric event” might be the most plausible explanation, at least according the FBI’s current publicly available understanding of the incident.