I had someone I knew from two consecutive jobs commit suicide. We'd routinely cross paths through meetings, water cooler chats etc. Nice guy, smart engineer. Capable of handling stressful incidents without batting an eyelid and spotting the shorted path to the best resolution. The kind of engineer you'd be lucky to have on your service team.

He grabbed me for a lunch time meal about a week before he committed suicide, wanted to chat about my faith. These conversations happen from time to time, especially working in tech which seems to bias towards atheism, so I didn't think anything of it. It was a type of conversation I've had dozens of times over.

In hindsight, of course, it was obvious he was looking for help. I can rationally tell myself over and over again that there was no possible way I could have known, but I highly doubt I'll ever convince myself of it.