A tattoo or scarification is an example I would think you would agree with. I would hope you agree that these are not (usually) due to mental illness, they also dont benefit others, and do in fact cause harm to ones self. So how do we handle that in your model? Its clearly self-harm, but in your model its an exception somehow? why?
As I mentioned before, it’s not only mental illness, It might be an emotional distress, underlying suffering or a coping mechanism, etc. The result is a net negative for the individual and society.