Evangelium Vitae #9
But God cannot leave the crime unpunished: from the ground on which it has been spilt, the blood of the one  murdered demands that God should render justice. (cf Gen 37:26, Is 26:21, Ez 24: 7-8). From this text the Church has taken the name of the ‘sins which cry to God for justice’, and, first among them, she has included wilful murder. For the Jewish people, as for many peoples of antiquity, blood is the source of life. Indeed, ‘the blood is the life’ (Dt 12:23), and life, especially human life, belongs only to God: for this reason, whoever attacks human life in some way attacks God himself.
Cain is cursed by God and also by the earth, which will deny him its fruit (cf Gen 4:11-12 ). He is punished; he will live in the wilderness and the desert. Murderous violence profoundly changes man’s environment. From being the ‘garden of Eden’, (Gen 2:15) a place of plenty, of harmonious interpersonal relationships and of friendship with God, the earth becomes ‘the land of Nod’ (Gen 4:16), a place of scarcity, loneliness and separation from God. Cain will be ‘a fugitive and a wanderer on the earth’ (Gen 4:14): uncertainty and restlessness will follow him forever.
And yet God, who is always merciful even when he punishes, ‘put a mark on Cain’, ‘lest any who came upon him should kill him’ (Gen 4:15). He thus gave him a distinctive sign, not to condemn him to the hatred of others, but to protect and defend him from those wishing to kill him, even out of a desire to avenge Abel’s death. Not even a murderer loses his personal dignity, and God himself pledges to guarantee this. And it is precisely here that the paradoxical mystery of the merciful justice of God is shown forth. As Saint Ambrose writes, “Once the crime is admitted at the very inception of this sinful act of parricide, then the divine law of God’s mercy should be immediately extended. If punishment is forthwith executed on the accused, then men in the exercise of justice would in no way observe patience and moderation, but would straightaway condemn the defendant to punishment. … God drove Cain out of his presence and sent him into exile far away from his native land, so that he passed from a life of human kindness to one which was more akin to the rude existence of  a wild beast. God, who preferred the correction rather than the death of a sinner, did not desire that a homicide be punished by the exaction of another act of homicide.
