Hi all!
We (orthodox Jews) infer the ban on suicide from Genesis 9:5 ("And surely your blood of your lives will I require..."). The Jewish attitude towards suicide, as opposed to selfless martyrdom, is rather complicated. On the one hand, while voluntary and premeditated suicide is considered to be reprehensible and an affront to God, all suicides are
a priori assumed to have done so without the necessary premeditation, whether from pathological depression, not being in possession of his faculties, or from having been under duress, etc. until it can be proven otherwise. "Duress" mainly means the necessity of having to kill one's self (or consent to your being killed) rather than violate one of the three sins that a Jew must never commit even at the cost of his/her life (murder, adultery/incest, and idolatry), or to prevent being captured alive by heathen if this would involve a desecration of God's Name and would, in itself, be a sanctification of God's Name. The suicide of King Saul is a good example of the latter (our Sages do not condemn King Saul, "because he knew that the Philistines would do with him as they pleased, and put him to death"); the suicide of Samson is another. The mass suicides which took place during the Middle Ages in order to avoid forced baptism, or the March 1190 mass suicide by the Jews of York, England (see
http://www.bispham2.freeserve.co.uk/castles/york.htm, scroll down to the last 3 paragraphs), are generally deemed to be martyrs who died in the sanctification of God's Name.
Jews who (God forbid!) commit voluntary, willful and premeditated suicide (and who are ruled as such by an orthodox rabbinical court; see above) are not to be mourned for; i.e. surviving first-degree relatives do not observe the usual Jewish mourning rites & practices. There is a custom that the kaddish prayer be said for 12 months (it's usually said for 11 months only) for a willful suicide, the assumption being that his/her soul needs the extra month of having kaddish recited for it for an additional month.
The above being said, as an orthodox Jew, I strongly oppose "assisted suicide" a la "Dr." Jack Kevorkian. What he does is cold-blooded murder. Judaism teaches that we do not enjoy proprietary rights over either our bodies or our souls, neither is ours to do with as we please. (Orthodox) Judaism does not hold "quality of life" to be a valid consideration in this context. We believe that ALL life is precious & holy and that such as we are incapable of deciding when "quality of life" is such that ending the life in question is warranted.
While nothing may be done (under Jewish law) to shorten the life of an ill person, if someone is really terminal & is really at death's door (or "has one foot in the next world" as we say), one need not, should not, take measures that would detain a soul that has already begun its journey to the next world. There's an account in the Talmud of a great sage who was terminally ill & was clearly dying. His students were gathered around him praying for him. Only their prayers were keeping their teacher's soul in this world. The teacher's maidservant understood this. She took a glass dish, held it up & then let it go. It fell to the floor & shattered. The sudden, sharp noise startled the students & interrupted their prayers. In that instant, their teacher died. The Talmud says that what the maidservant did was praiseworthy & what the students were doing was wrong.
My wife has a cousin here in Israel whose wife gave birth prematurely to twin boys. One boy was much stronger than the other & spent many months in the premie ward in a local hospital. He is now a healthy toddler (thank God!). His twin brother was not so fortunate. He was very weak, had severe, multiple, developmental problems. The doctors gave him absolutely zero chance of surviving more than a few weeks even with the best of care, heroic measures, etc. The parents consulted a very prominent (orthodox) rabbi who is an expert in such matters. He ruled that given what the doctors said, that the infant should be fed/hydrated but that nothing else need be done. The boy died after only a few days in this world (may God bless and keep him!).
I believe that the general principle in (orthodox) Jewish law is that one need not be hooked up to lifesaving machines but that once hooked up, they may not be turned off or unhooked.
Be well!
ssv