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.

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