@fastjet45 I agree it wouldn't be spur of the moment however if he didn't know he'd be alone the tone of his voice should make no difference, that moment could have come anytime but the way it's been reported makes it sound like he was plotting to do something because he knew he was about to be alone.
I've suffered severe depression myself in the past which has reared it's ugly head after feeling better at times. I remember doing a medical questionnaire for a job and I purposely did not mention my history on this as I know the stigma that surrounds it. The job before that I only came out about it as I was being disciplined and I needed to tell the truth to save my job, work life after that admission was tough so it wouldn't surprise me if he was suffering he wouldn't want to tell anyone even more so being in a job where good physical and mental health is needed. Imagine not only having to admit depression but admitting that after years of hard work and getting your dream job you have to admit that even that isn't making you happy. Obviously though I can only give my opinion on what I felt and put that in to the shoes of someone in that job, I am by no means saying this was what was in his mind.