Not every mental illness affects your grasp of logic or perspective on reality. Obsessive compulsive disorder can be absolutely debilitating even though the person might completely understand the reality of their situation, know that they don’t actually benefit from and in fact are harmed by, for example, checking the windows and doors every night in the exact same way many times, but still be unable to stop themselves. They aren’t delusional, just prisoner to compulsions they can’t resist. I’m not suggesting that OCD is always severe enough to make someone consider euthanasia or is always resistant to treatment, but it certainly could be.

I could also see someone with substance abuse disorder that they’ve tried and failed to treat for decades making that decision (while sober after detoxing and withdrawal). I would want them to keep trying, because I know how great sobriety and how valuable a life built post addiction can be, but I wouldn’t consider them to be of unsound mind in that scenario.

I’m just listing examples in my family, but I’m sure there are others.