With mental illness people often seem to spend their whole lives fighting to survive, almost as if they are fighting this predetermination. Suicide can often be seen as an expression of free will to counter this. Maybe that self destruct mechanism is built in and gets expressed by difficult life circumstances. And yet, some people go through hell and survive, even flourish, while others are triggered by events that wouldn't effect others in that way.
I hesitate to say this, but... suicidality could be (to a certain extent, some circumstances would floor anybody) another negative feedback mechanism that nature puts in place to remove individuals from the gene pool. Yeah, this is harsh, but nature is harsh. So many of the things we do in life seem to attempt to go against nature - even actually surviving sometimes seems to be to try and thwart the ultimate entropy that claims us all. Again, its this sodding paradox that drives us.
Note: you'll see from many of my answers on subjects like these that my thoughts are nebulous and contradictory. I don't necessarily believe many of the things I put forward are true, they are merely things I think about. My mind is open and I think the confusion and contradiction that may come across in my posts is actually reflective of the nature of our understanding of such ephemeral things.