You shall not kill.
You have heard that it was said to the men of old, "You shall not kill: and whoever kills shall be liable to judgment." But I say to you that every one who is angry with his brother shall be liable to judgment.
"Human life is sacred because from its beginning it involves the creative action of God and it remains for ever in a special relationship with the Creator, who is its sole end. God alone is the Lord of life from its beginning until its end: no one can under any circumstance claim for himself the right directly to destroy an innocent human being."
-- CCC 2258