This is how I feel about it. None of us ask to be born (as far as we know), and if you don't want to keep going then you shouldn't be forced to. What sucks is the aftermath for everyone still alive that loves and depends on you.
Lost my best friend from college to this about a month ago. His life from his mid-30s on was a mess by just about every metric. He was looking at another 3-4 decades without much in the way of job skills and financial security, let alone the motivation to get himself out of bed every day to get it back together, in addition to what was probably some bad brain chemistry. I think he saw that reality in combination with his own misery and other issues he was facing and said, "no thanks- not doing it". Ended up having a psychotic episode a few months ago, followed by other issues and eventual a deep depression that ate away at him until he finally did the deed.
What I wonder is if those issues were always under the surface and really came to life once he started having real life issues. He could get pretty nasty in arguments, far beyond the point of what's a 10 on the anger scale for most of us. I wish he could've gotten the help he needed, but even then I'm not sure if he had it in him to the work at that point. Mental illness sucks.