good grief, that's horrible. I too know multiple people that were involved in my life that killed themselves and I always ask myself "what could I have done to prevent this?" and i suppose that is not fair to do to ourselves.
I'm nice to people for the most part and I would be willing to bet that you are as well.
That story about your ex is horrible and the friends that took off, well, you don't need them.
Yes, the "what if's" can be a real killer, but I suppose to a degree I have started to accept that I was at least able to brighten his world for a time - be it a short time.
You would think that realisation would set in after having had so many people "discard you" but I am a stubbornly eternal optimist and I always try and see the best in people - this is where I often fall short... I think. However, on the flipside of that coin, I never want to harden my heart... but I am realising that I need to see things for what they truly are and cut my losses, as you said.