I agree with you killing is killing even at war. The only reason I mentioned I don't know what to think when it comes to war, is because David fought Goliath with God and killed Goliath. So in a way, God was behind David's killing of Goliath. David asked God for help and then killed Goliath. See my point?
When it comes to suicide, unless one is not of sound mind, to me it is sin. Why? Because God is the author and finisher, He decides how many days you will live so if you take your life, you're taking matters into your own hands and totally disregarding His plan for you. In other words, you are playing god; taking that decision away from God. Is that OK to do?
My frame was not hidden from you when I was made in the secret place. When I was woven together in the depths of the earth, [16] your eyes saw my unformed body. All the days ordained for me were written in your book before one of them came to be.
(Psalm 139:15-16 NIV)
[19]Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; [20] you were bought at a price. Therefore honor God with your body.
(1 Corinthians 6:19-20 NIV)
Does doing harm to your body fall under honoring God?
Then there's the issue about forgiveness that you mention.
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
(1 John 1:9 NIV)
OK, God will forgive if we confess our sins. Usually that happens after we commit the sin, so obviously if you commit the sin of murder and you're the victim, you will not get to confess your sin. (I am still referring to one of sound mind. I will return to address one who is insane later.)
Do I take any pleasure in the death of the wicked? declares the Sovereign LORD. Rather, am I not pleased when they turn from their ways and live? {24} "But if a righteous man turns from his righteousness and commits sin and does the same detestable things the wicked man does, will he live? None of the righteous things he has done will be remembered. Because of the unfaithfulness he is guilty of and because of the sins he has committed, he will die. {25} "Yet you say, 'The way of the Lord is not just.' Hear, O house of Israel: Is my way unjust? Is it not your ways that are unjust? {26} If a righteous man turns from his righteousness and commits sin, he will die for it; because of the sin he has committed he will die. {27} But if a wicked man turns away from the wickedness he has committed and does what is just and right, he will save his life. {28} Because he considers all the offenses he has committed and turns away from them, he will surely live; he will not die.
(Ezekiel 18:23-28 NIV)
So if you live a godly life and then sin and don't get a chance to turn away from that sin, then all the good you did will be forgotten. And if you kill yourself, you don't get a second chance.
Just as man is destined to die once, and after that to face judgment
(Hebrews 9:27 NIV)
So if you kill yourself, you die having committed a sin, and then comes judgment.
That last verse actually addresses my line that you quoted "suicide is one of those things we as humans cannot make judgments on--since depression is a disease--and perhaps we'll just have to let the true Judge make that call on Judgment Day" because God will know how to deal with each one. If it happens out of madness, I believe God is a merciful God. But if you were of sound mind and reached that crossroad where you had a choice to go either
- the way of pain and trust that the Almighty, All-Knowing, Ever-Present God who has a plan for you (Jer 29:11) and who has asked countless of times to call unto Him in your time of need, who has shown you in His word and I'm sure in your life that He is Lord and no one that comes to Him is ever disappointed...believing He will see you through; or
- ignore God and His will for your life; forget all that you know of His Person and his ability to put all the pieces of your life together. In other words, you lose faith and doubt that He will bring you through ("And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him." (Heb 11:6 NIV) and ignore his command "Call unto me"...but decide to do what you in your human mind (not Spirit led) believe is the thing to do.
In other words, just like God gives us a choice whether to accept Him in our lives or not, at that crossroad, you have a choice whether to reject God's plan and will for your life and do your own thing, or whether to accept his plan for your life (even if it means it could be a journey as trying as Job's) and His help believing in the promises in His word. ("No temptation has seized you except what is common to man. And God is faithful; he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can stand up under it." (1 Cor 10:12 NIV))
An interesting read on the Bible on suicide:
http://www.christiananswers.net/q-dml/dml-y038.html