This isn't Russian assassins. They kill specific notable people, not functionaries. Nor is this about not being able to talk about work. Working in these fields becomes your life. You talk about it all day. They don't want to talk about it more, certainly not at home.

This is about over-qualification in a narrow field. These are people for whom there is only the one job. Then ... something happens that threatens that existence. Think of a young airline pilot being diagnosed with an eye problem. His flying days are over. His entire experience is meaningless to any other jobs and he must start from scratch: a 40yo with no qualification beyond highschool.

Dig into these people and I think you will find something related to their security clearances. Something bad, or something totally innocent, that threatened their clearance. They are faced with a professional reset, something that will impact them and their family in a negative and permanent way.

From the article: Sergeant Kevin Stallings ... He took his own life the month before he’d have turned 37. ... grew up in a military family ... enlisted in the Air Force right out of a Virginia high school ... earned a degree in intelligence analysis

So he had no qualifications outside of intelligence. He had little to no perception of any life outside his field. So anything threatening that job, real or imagined, would be instantly existential.