You ask some very profound questions there @Volomori83 and it encourages the kind of conversation I came to this forum for. Like you, I ponder such things. Because, like you, I wonder if I must be ill.
How do we define 'ill'? 'It's similar to asking 'what is normal?' We humans love to (perhaps even need to) stick labels on things. We like to categorise. Clearly there are very clear cut examples of mental illness, illnesses that require medical intervention, but when you move away from those and move towards 'normality', then things aren't so clear. Illness exists on a spectrum, I think that most (if not all) of the things we categorise do. Is (some) illness simply something that is outside what most people think of as normal? With no other rationale than that? I don't know.
FWIW, I don't think I'm ill for having suicidal intentions. But then I would think that wouldn't I - I'm ill! (The paradox, catch 22) In fact, my thoughts feel coldly logical, neither deranged nor delusional. It feels like a well judged decision based on the evidence available to me. I've mentioned this in my other posts but, to be brief, I feel that I've done enough now and there's little left that I feel motivated to do. I feel that I'm simply ticking over, treading water. Yes, I accept that could be depression but, like you, I don't feel it is. Moreover, it's immaterial. The will to do something about it (if it were depression) has gone. Whatever the reason for it, I live with it on a daily basis and I see little reason to continue like this.
Why do people feel this way? Again, I don't know. I could talk about personal fulfilment - we either lack it or, conversely, have reached it. I could talk in terms of society; dehumanising, the treadmill etc. Perhaps modern life isn't good for us? Maybe we are living too long or maybe just living to long - that is, in search of something that will captivate us and give us reason (hence my user name incidentally, a play on words) Maybe we just reach a point where we say 'done now, nothing left' I recall that one of the philosophers (I've read some but by no means many) remarked - and I paraphrase - that suicide was an entirely rational decision; who would, faced by life's absurdity, not want to end it.
If I may sidetrack (not in an attempt to deter you but simply to convey something I found in reading this forum), I too have read the stories here. I have been incredibly moved by many of them. The suffering that people have to endure, both physical and mental, gives me good reason to be understanding of a decision to ctb. I have also read the stories of the attempts; the courage and determination people show is enviable. I have also followed the links to pictures and videos of both successful and unsuccessful attempts, and the latter can be horrifying. In the former, I saw something that went to my soul - I saw life gone. It made me realise what a huge decision it is to ctb. I equally must admit to a sadness; to end your life, the only one that you have, is not something I'd wish on anyone. I saw the end of everything that person ever was - yes, I felt a sadness.
When my younger brother died (naturally, a genetic illness) I viewed his body in the mortuary and I saw a similar thing, but had the opposite reaction - I was almost calmed. That corpse was not him (that is, not my brother as I knew him) My brother had gone and I found peace in that. Not because his suffering was over but because my own fear of him being DEAD was unfounded. I've no idea where my brother was but I felt comfort in feeling that the corpse wasn't him.
I've no idea what to make of those differing reactions but I have, for now, stepped back from the edge. I want to think a little more and be extra sure before I throw my hand in.