Catechism of the Catholic Church §2302 — excerpt
By recalling the commandment, "You shall not kill," our Lord asked for peace of heart and denounced murderous anger and hatred as immoral. Anger is a desire for revenge.
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: St. Thomas Aquinas, STh II-II, 158, 1 ad 3