There´s nothing sinful in suicide, it depends on the situation but in almost every case the one who commits suicide has got a good reason for doing so.
Some examples: Harakiri that the samurais commit because of shame(?); end suffering; to save another persons life; because you are depressed for some reason; ect.
And you who says it´s cowardly to commit suicide, how come?
Sure, the one who does it because he/she is depressed could get help in another way, but during a mental breakdown, when you feel nobody can help you, nobody cares about you, nothing´s gonna make things better, I can understans why people commits suicide. And it´s nothing wrong with the person doing so, it´s wrong somewhere in the persons surroundings, and if you can fix that before the person commits suicide, you´d probably make sure he/she regaines the will to live.
Edit: There´s no such thing as a sinful deed. There´s only good, bad and selfish deeds, and selfish deeds doesn´t neccesary have to be bad deeds.
We´ll get neither to heaven, nor to hell, so why care about every deed you make, you can´t make everyone happy can you?, so just live your life and don´t care so much about the rest of the world.
Peace out!