What is puzzling to me is not that he may’ve had depression or other mental health issues. Psychologists are human and suffer from the same things we all do. It’s that he had experience and access to resources - his job was to help people who are exactly that - suicidal - and yet he did not use those resources himself.
I am not in any way blaming the victim here, I am pointing out how invasive depression and some mental health issues can be, that someone with all of his professional knowledge can’t even follow through on conventional advice.
I also know a funeral director who died by suicide. I felt similarly, that he had seen the consequences of suicide professionally, yet still could not pull himself out of the depth. (Although in his case, it appeared to be “out of the blue.” No history of depression, no one saw any signs etc)