Suicide was for centuries considered a sin not necessarily because it was considered murder, but because it was seen as a rejection of God.--a sin of despair. The suicidal person was in essence saying he or she is in a state of hopelessness beyond the grace of God. Hopelessness in the midst of suffering is the very nature of Hell or the Inferno. (See Dante's Inferno, cantos IV and XIII). Thus the suicidal person is in a sense already in a state of damnation before the act itself. Dante himself feels pity for the Christian suicides.
I remember as a teen reading Dante for the first time and being taken aback by the thought of suicides being damned, It seemed the height of injustice to me for a person to have lived an utterly painful existence and then to end it only to find themselves in a worse place! The notion of damnation and suicide gets picked up again literature specifically in Hamlet as Ophelia's apparent suicide creates some consternation as to whether she deserves a proper Christian burial. Moreover, Hamlet himself seems to toy with suicide when he wishes that "the Everlasting had not fixed his canons against self-slaughter." Later he decides against suicide in his famous "to be or not to be soliloquy" in which he says;
When he himself might his Quietus [death] make
With a bare Bodkin [knife] ? Who would these Fardels [troubles] bear,
To grunt and sweat under a weary life,
But that the dread of something after death,
The undiscovered Country, from whose bourn
No Traveler returns, Puzzles the will,
And makes us rather bear those ills we have,
Than fly to others that we know not of.
Thus Conscience does make Cowards of us all,
That we may travel to a worse place apparently troubles the will; yet it is the very notion of will that has changed many modern Christian perceptions of suicide. For a person to be culpable in terms of a sin, the will has to be employed. One must employ the will in willingly going against God.
Today, unlike in the past, we understand that mental illness, very often the root of suicide, is a disease that undoubtedly impairs the will. People with severe mental illnesses are not considered culpable for their sins --either on earth and we presume in heaven. It is the belief of many Christians then that likewise such a mental illness of deppresion nullifies suicide as a sin. Few Christians/Catholics that I know believe that an act of suicide automatically damns a person.
Ultimately Chrisitans have to take faith that somehow perfect justice and perfect mercy , a quality only attributable to God, will be in force in determining our place in eternity.
It is my personal hope that no one--none--suffer eternal damnation--yet I acknowledge that the will of God in blending mercy with justice will have the final say.