For I tell you, if your virtue goes no deeper than that of the scribes and Pharisees, you will never enter the Kingdom of heaven. (Mt 5:20)
The righteousness of the Pharisees is that they shall not kill; the righteousness of those who are destined to enter into the kingdom of heaven is that they shall not be angry without cause. Not to kill is, therefore, the least commandment; and whoever shall break that shall be called the least in the kingdom of heaven. But whoever shall fulfil that commandment not to kill will not necessarily be great and fit for the kingdom of heaven; but yet he has ascended to a certain degree. He will be made perfect, however, if he refrains from anger without cause; and if he shall do this, he will be removed much further from the guilt of murder. And, therefore, he who teaches that we should not be angry, does not destroy the law which forbids us to kill, but rather completes it; so that we preserve our innocence both outwardly when we do not kill, and in the heart when we refrain from anger.
– From: St Augustine, Bishop, Book 1 on the Sermon of the Lord on the Mount, Ch. 9, from An Approved English Translation of the Breviarium Romanum, Burns & Oates, London, 1964