Is it possible to be sane and commit suicide?

I have heard of two people recently, one of my not so close friends and somebody from my apartment building, I know for a fact the apartment one was completely insane like he thought he was going to go to aliens or something crazy. the not so close friend I believe was out of depression. I came to thinking though. It is the nature of life to live. That is the main purpose of every single living organism. So in order to commit suicide it would be completely unnatural and thus insane to do. (Note: Suicide and Martyrdom are two different things and I just want to make sure I don't see martyrdom as an objection, my friend offered this tonight as an objection and i believe it is a flawed objection for it is done with the hopes that the human species will move forward learning from it and thus in line with nature.)