As I said, ordinarily I would be in absolute agreement with you. But a case like this makes you realise that , as with most things in life, it is not a black and white issue. Lots of horrible shades of grey.
I don't know why it should seem worse that a person would choose to end their life because of mental health problems, rather than because of incurable cancer or any other disease. But deep down, for some reason, it just does feel worse. Maybe its because we (I?) have the idea that mental health problems should be treatable/manageable, while accepting the fact that cancer or many other diseases very often are not. I really don't know. I have very little knowledge or experience of mental health problems, so I am going to shut up about it now.