Not only is suicide a sin, it is the sin. It is the ultimate and absolute evil, the refusal to take an interest in existence; the refusal to take the oath of loyalty to life. The man who kills a man, kills a man. The man who kills himself, kills all men; as far as he is concerned he wipes out the world. His act is worse (symbolically considered) than any rape or dynamite outrage. For it destroys all buildings: it insults all women. The thief is satisfied with diamonds; but the suicide is not: that is his crime. He cannot be bribed, even by the blazing stones of the Celestial City. The thief compliments the things he steals, if not the owner of them. But the suicide insults everything on earth by not stealing it. He defiles every flower by refusing to live for its sake. There is not a tiny creature in the cosmos at whom his death is not a sneer. When a man hangs himself on a tree, the leaves might fall off in anger and the birds fly away in fury: for each has received a personal affront. Of course there may be pathetic emotional excuses for the act. There often are for rape, and there almost always are for dynamite. But if it comes to clear ideas and the intelligent meaning of things, then there is much more rational and philosophic truth in the burial at the cross-roads and the stake driven through the body, than in Mr. Archer's suicidal automatic machines. There is a meaning in burying the suicide apart. The man's crime is different from other crimes -- for it makes even crimes impossible.
GK Chesteron, Orthodoxy
I know that there are a number of people who are grieving over Robin Williams suicide--it seems that every major celebrity who dies, and especially who dies early, invokes a bit of national mourning. It is a tragedy, and depression is an unfortunate thing to struggle against.
With that said, there is something far, far worse than depression even, something for which depression is at most a symptom, though something which may exist without depression. That is despair. There is the suicidal depression which preys upon the mind and the emotions, but it pales next to that deep and dark despair which preys upon the very soul. The man who is depressed may yet find hope: but he who finally despairs has rejected hope and so is lost.