[M]ortal sin destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. In mortal sin the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end. As such, the sin is mortal by its very object whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery(Catechism of the Catholic Church # 1855-1856).
Many people today scoff at the idea that mortal sin is a turning away from God. They doubt that people directly intend to turn away from God, as if the fornicator or the murder or the thief would say, “I hate God and so I am going to turn away from Him by sinning.”
That is not what catechism says, however. Rather, it says that our preference for an inferior good to God by a grave violation of His law is what turns us away from Him.
It says that in mortal sin we set our will upon something we know to be incompatible with our ultimate end. Although our first thought may not be that we are rejecting God, we set our will on something incompatible with God. In so doing, we are preferring something or someone to God.
This poisons our heart if we do not repent because we feed a desire in our heart for what is not God and we starve our heart from Him and what He offers. Soon enough we prefer the darkness to the light. We prefer the trinkets of this world to God and come to regard Him as a thief who comes to take what we want and keeps us from doing what we want to do. God becomes our enemy.
If we die in this state, the warmth of God and Heaven seem overwhelming, wrathful, and like a consuming fire. We cannot endure and so we turn away finally and permanently to a place that we strangely prefer, but which is hellacious because it is not that for which we were made. It lacks the one thing necessary: God.
And this is the judgment: the light has come into the world, and people loved the darkness rather than the light because their works were evil. Everyone who does evil hates the Light and does not come into the Light (John 3:19-20).
In mortal sin it is not that we directly turn from God — at least not at first — but that we turn to the lesser things of the darkness and come to hate Him who is the Light.