People without mental illnesses kill themselves for various reasons. Look at that Castro guy that held those girls captive and raped them for years. He got caught and put in prison, so rather than serve his time, he offed himself. No mental illness needed for that.
Even if the open windows of science at first make us shiver after the cozy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own - Bertrand Russell