Once again the authorities dropped the the ball. This time it looks like it was the military that effed up. Perhaps by design? Major mental health risk with a violent threat, and they didn't take his weapons away? Insane.
He was almost certainly prescribed SSRIs or anti-psychotic medication. If he stopped suddenly, there would be severe mental problems
Also, the psychiatrist should have requested all guns be removed from his house. (Whether is not he would comply would be a different matter.)