I am a committed Christian, ordained minister, and chaplain. I love God with all my heart, and would do my best to obey any directive from God.
However, i also suffer with major depression. I can assure you that it is very possible for the illness of depression to warp your thinking in such a way as to cause you to think suicide is the answer, even if you are a believer. It is the same as someone who is in immense pain and deciding to stop treatment and allow themselves to die. There is such great pain in depression, beyond any physical pain you can imagine.
This pain, like all pain and sorrow, is rooted in sin, but not necessarily my sin. I did nothing that led to this illness. I was born with it, I believe. I know it began before I was old enough to go to school. There were times in my past that I considered suicide. I thank God that He took those thoughts away. But I also know how easily they could return if I lowered my guard in this area.
Only God can declare whether this person is saved. Suicide is never God's will, but we all sin. The only unforgivable sin is blaspheming the Spirit. That means suicide is NOT an unforgivable sin.