"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. 'To desire vengeance in order to do evil to someone who should be punished is illicit,' but it is praiseworthy to impose restitution 'to correct vices and maintain justice.'
If anger reaches the point of a deliberate desire to kill or seriously wound a neighbor, it is gravely against charity; it is a mortal sin. The Lord says, 'Everyone who is angry with his brother shall be liable to judgment.'
-- CCC 2302