Depression and psychosis are actual classes of mental illnesses that can include suicidality. Malingering and Factitious Disordered presentations are insincere (but sometimes convincing) claims of suicidality with the intention of obtaining something from someone. Terminal illness-related suicidality is many times fairly rational and a sign of neither mental illness nor "cowardice" as some folks here have suggested.
Criminals who commit suicide when they're finally caught or convicted commonly generate pretty negative feelings about them, so I could certainly understand the impulse to call these folks "cowards," but they don't tend to exactly be "mentally healthy" individuals either.
Just a few of my thoughts on the topic.