Suicide for the unbeliever is the worst thing a person can do, apart from rejecting God's provision for salvation and the forgiveness of sins in Jesus. This is so becae there are no second chances because, ". . . it is appointed for men to die once, and after this comes judgment" (Hebrews 9:27).
For the Christian, I would deal with suicide with three main points:
(1) Suicide is sin. God forbids murder and taking our own life is murder. It is also sin because it fails to take into account the assurances of God's grace and mercy, of the compassionate High Priesthood of our Lord Jesus, and because it believes that there is no way out, but to sin. None of these are true. Taking one's life is to disobey God, and disobedience is sin. Suicide is also sin because it seeks to deal with the pains we face in life by placing that pain, guilt, and sorrow on those we leave behind.
(2) While suicide is sin, it is not the unpardonable sin. The unpardonable sin is attributing the miraculous work of the Holy Spirit in Jesus to Satan (Mark 3:20-30).
(3) While suicide is sin, it is more easy to understand why a Christian would wrongly conclude that suicide is better than suffering the trials and afflictions of this life. This is not to understate how terrible this choice is, and how destructive to one's testimony and to one's claim that God is sufficient for our every need. But a Christian is assured of going to heaven when they die (even if that be by suicide). When one is overcome by depression and is not thinking clearly, suicide "appears" to be the easy way out, which is one of the reasons why it is sin.
(4) There were genuine believers in the Bible who wanted to die, men like Elijah and Job, but the Scriptures that describe these men's despair and suicidal desires make it clear that they were wrong. Later events make it clear that "bailing out of life" would have been a very bad choice.
(5) God may bring us to a point of total despair and frustration so that we will give up trying to live this life in our own strength and cast ourselves upon Christ.