Ignoring the 'sin' aspect of it, if one considers Life to be a gift then suicide is a rejection of that gift. I also think we grow from each experience we have in this existence, and ending it early handicaps one in the next existence.

Don't get me wrong, I watched my uncle die from prostrate cancer, and I won't go out that way. But, excepting a terminal situation, suicide is usually a permanent solution to a temporary problem.

I don't believe in "Hell", I just think those who were that sort simply cease to exists upon their Judgement. A merciful God would not torture His children endlessly, it would be kinder to just end their existence.