[portofcall]

This I just wrong from the start. If there’s a single overarching definition of mental illness, it’s “clinical significance” and not deviation from the norm. Of course it’s not that simple either, but it is a common thread throughout all of them.

[Karunamon]

This only moves the definition down one level. If clinical significance (in general, not just in psychology) isn't based on deviation from a norm, what is it based on?

[portofcall]

The degree to which something interferes in someone’s life, their ability to function, etc. If you’re suicidally depressed, or psychotic, it’s a clear distinction for obvious reasons. It’s not about comparing to some standard, you can believe strange things, be unhappy, and so on, but if it starts to make you unable to live your life, it might be a problem.

[Karunamon]

There's an argument to be made that engaging in murder-suicide is of absolute interference to one's life and ability to function.