Catechism of the Catholic Church

2259 In the account of Abel's murder by his brother Cain, ⁵⁷ Scripture reveals the presence of anger and envy in man, consequences of original sin, from the beginning of human history. Man has become the enemy of his fellow man. God declares the wickedness of this fratricide: "What have you done? The voice of your brother's blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother's blood from your hand." ⁵⁸

2260 The covenant between God and mankind is interwoven with reminders of God's gift of human life and man's murderous violence:

For your lifeblood I will surely require a reckoning.... Whoever sheds the blood of man, by man shall his blood be shed; for God made man in his own image. ⁵⁹

The Old Testament always considered blood a sacred sign of life. ⁶⁰ This teaching remains necessary for all time.

2261 Scripture specifies the prohibition contained in the fifth commandment: "Do not slay the innocent and the righteous." ⁶¹ The deliberate murder of an innocent person is gravely contrary to the dignity of the human being, to the golden rule, and to the holiness of the Creator. The law forbidding it is universally valid: it obliges each and everyone, always and everywhere.

2262 In the Sermon on the Mount, the Lord recalls the commandment, "You shall not kill," ⁶² and adds to it the proscription of anger, hatred, and vengeance. Going further, Christ asks his disciples to turn the other cheek, to love their enemies. ⁶³ He did not defend himself and told Peter to leave his sword in its sheath. ⁶⁴