Yes I believe that STBX is suffering from depression. When I first joined the boards, one of my guiding lights was the thought that STBX was sick, and our vows included sticking together through sickness and in health. Of course, I have wavered on that somewhat, because obviously not all depressed people make these kinds of decisions, so I still feel there is a fair amount of individual choice at play.
In fairness, my STBX has never seen a counselor and has never been diagnosed (to my knowledge), so I'm just indulging in some armchair psychology here. Soon after BD, when I was scouring the internet I read several articles that explained how depressed people behave when in affairs and they described STBX to a tee. He does have quite a number of risk factors including sleep issues and dealing with some pretty disturbing subject matter at work.
I know there is absolutely nothing I can do about any of this. At least not now. I do have some curiosity about what will happen when he realizes I am not the source of all his woes. Or who knows? He may just blame me for everything for the rest of his life.