Catechism of the Catholic Church §1759 — excerpt
"An evil action cannot be justified by reference to a good intention" (cf St. Thomas Aquinas, Dec. praec. 6). the end does not justify the means.
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.