Why isn’t God asking Abraham to murder his son a moral contradiction?
The answer is contingent upon a correct understanding of the word murder. Murder is taking the life of another without the moral authority to do so. Some killing is morally licit—e.g., the killing of an assailant in self-defense of oneself or one’s family (CCC 2321).
In addition, even though the Church strongly discourages capital punishment, the Church distinguishes between abortion, which is always gravely wrong (CCC 2322-23), and capital punishment, which is not an intrinsic evil (CCC 2266-67).
The killing of Isaac would not have been murder, first of all, because he inherited original sin and also had committed personal sins (Rom. 5:12). Indeed, “the wages of sin is death” (Rom. 6:23). Also, God is the Lord of life and death (Deut. 32:39, 1 Sam. 2:6), and therefore it’s under God’s sovereign authority to determine when and how someone’s life begins and ends.
In this light, Abraham would not be guilty of murder himself if he had killed Isaac, as he would have been acting on the authority of a legitimate moral authority—namely, God.
God tested Abraham to see whether Abraham truly trusted him. It was a severe test because God had told Abraham that the world would be blessed by his descendants through Isaac (Gen. 12:1-3, 17:15-21). Abraham passed his test of trust because “he considered that God was able to raise men even from the dead; hence he did receive [Isaac] back, and this was a symbol” (Heb. 11:19).
That symbol, of course, was Jesus Christ, as the Father sent his only begotten Son to die for our redemption and salvation.