Sin is natural to anyone who is not born of the Spirit.
Does the nature of sin change from one person to the next? Sin is just as wrong no matter who commits it. If we are born of the Spirit, we ought not to sin, but if we do sin it is evil and ghastly.
Nature is the creation of God, governed by the laws of God. The law of God commands physical earthly things, such as the earth, gravity, magnetism, elements, etc.
If I threw a ball in the air and it kept going up instead of falling, I would call that unnatural. The ball is disobeying the laws of physics. Since the laws are fixed by the Creator of all things, the ball ought to fall in accordance with those laws (unless their is a supernatural occurence).
Human behaviour is governed by the law of God as well. The law says that humans ought not to covet, steal, murder, fornicate, defraud, or lie. Thus, when humans do these things, they are acting contrary to what God has fixed in nature as appropriate behaviour.
Imagine that gravity suddenly stopped working everywhere. Instead of resting against a normal force, every object on earth was suspended in air. I would regard that as quite an unnatural state of affairs.
Thus, when the Catholic philosopher sees a world full of sin, they see mankind acting against what God has fixed. Man breaks laws by sinning and does what is unnatural. Trees act precisely according to nature. They are perfect just by being trees.
Man has become like a brute, not doing what man ought to do. Man ought not to sin. When someone does something that they aren’t supposed to do, they are acting unnaturally. Thus, the world of men exists in an unnatural state of affairs. All people are breaking the law of God and this is pure madness. Nature is orderly, man is disorderly.
Praise be to Christ in Whom all things are restored.