Catechism of the Catholic Church §2268 — excerpt
The fifth commandment forbids direct and intentional killing as gravely sinful. the murderer and those who cooperate voluntarily in murder commit a sin that cries out to heaven for vengeance.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: GS 51 # 3