Heard on the news he'd lost the privileges of his license some time before.
A pilot who is suspended or facing termination must unfortunately be viewed as a potential security threat these days. Many U.S. carriers now have a quiet policy of deadheading these crewmembers on another airline to lessen the chance of an inside job of commandeering an aircraft.
Auburn Calloway at FedEx and Gamal El-Batouti at EgyptAir were both facing termination hearings when they attempted to crash their company aircraft (El-Batouti was successful with SU990).
Chris Phatswe took up an Air Botswana ATR-42 and commited suicide with it in 1999:
1999 Air Botswana incident - Wikipedia, the free encyclopedia
If the security had worked he wouldn't have been able to get near the airplane. He was a suspended employee. Plus, even an active crew shouldn't be able to gain access to an aircraft that was supposedly secured for the night.
Some fairly simple but effective measures restrict access to an unattended aircraft where I work. A few years ago TSA 'Tiger Teams' were testing overnight aircraft security, seems like one of them was using a TAT or AOA probe as a step to gain access to an overhead hatch and damaged several RJ aircraft.
I don't know about this particular type of aircraft, but I happen to have two keys in my possession that would enable me to unlock two fairly common types of aircraft, one 'large' and one 'small.' All of the aircraft of each type share a common key, and I have the keys because sometimes I flew those types.
I don't even have access to a Boeing or Airbus ignition key after hours.
The scenario of a pilot murdering a spouse or domestic partner has played out a few times. The most infamous case in my view is the gruesome Woodchipper Murder:
Helle Crafts - Wikipedia, the free encyclopedia