There's two (well, more than that, really) sides to the suicide coin.
1. You have men like this. Men who have seen that which they have wrought, and found a life well spent. Rather than wither away and die as you say, these people deserve an 'easy out.'
It think people seriously overlook the right to simply refuse futile and degrading extraordinary methods to preserve life beyond its natural conclusion. We have a lot of options for pain management that provide a useful alternative to taking a shotgun to one's face.
3. Also, you have those who genuinly have something wrong with their mind that pushes them to it. You can't blame someone for something external pushing it down on you like that.
Not just something wrong with their minds - there are people who have something wrong with their surrounding culture. There are many sad situations where "something external" is a social prerogative that condemns the disabled as being burdensome and unworthy of life.
4. Finally, unless there are more I'm too tired to think of, you have those that go for a good cause. The good soldier diving on a grenade. Sacrificing one's self for the good of many, etc - the true altruistic finale.
That doesn't qualify as suicide. That's an altruistic act of heroism that happens to have the undesired outcome of dying. Likewise, killing in self-defense is not murder. It is an act to preserve one's own life from an aggressor with the otherwise unwanted outcome of killing.