Half the time people's depression is purely situational. They go to the doctor and, being the lucrative path for the doctor and the drug companies, get on medicine for a chemical imbalance they don't have, end up becoming addicted, and have even worse problems when trying to get off of it.

As for 'mentally ill' people in general (excluding depression), I'm not sure if suicides under those circumstances are the cause of the treatment, and not the illness. The delusion-fests of exorcisms cause injury and death from the 'treatment' itself. I'll admit that sometimes for some people the only alternative is exorcism. They're bought into the delusion that they're occupied by evil spirits, and usually through the self-reinforcing delusional practice of exorcisms they become 'well again'. If the problem is psychological then maybe it can be 'cured' psychologically.