Agreed — that or actual brain illness like tumor or Alzheimer’s, if found. But he should still stay behind bars for the rest of his life. Surely such illnesses carry forewarnings, and the prosecution should certainly subpoena his medical records to see if he sought any kind of treatment. One hopes the security cameras will reveal if he left to speak to the manager or to get his gun from the car.
He should face the music or consequences for what he did. And that should be spending the remainder of his life in prison.