What have you done? The voice of your brother's blood is crying to me from the ground. And now you are cursed... Gn 4:10-11
God's disapproval and condemnation of taking another's life are clearly expressed in these words. God is the creator of life, and he entrusts it to us -- not freehold, to do what we like with it, but leasehold (and not always 99 years?), to use it under his direction in the service of man.
Human life is not in the same category as animal life. Because God made man for fellowship with himself, and because man is made `in the image of God' -- with a rational mind, a sense of moral values, and the ability to perceive the love of God and respond to it -- man's life is unique. He is of particular value in God's sight -- of sufficient worth for Christ to endure the Cross. The Cross is the measure of man's worth to God, who sees every man potentially as a son.
Here is a patient who is a drain on us; his disease is unpleasant, and he is useless to society. So was Lazarus, the sick man at the gate of Dives -- and yet God judged him worthy of eternal life (Lk 16:22). Or here is a victim of prolonged suffering. If he were an animal, compassion would compel us to `put him down'. But he is a man, so our principle must be to relieve his suffering by every other available means. Among these are modern drugs, human friendship and the message of the love of God.
I believe in one God... The Lord and giver of life.
Further reading: Gn 4:1-10. Mt 22:36-40