Nature, as a principle of motion, determines natural operations. From this perspective, goodness is the completion of a natural operation according to the nature of the particular creature. This is why we consider deformities in humans and other animals to be defective or, to put it more strongly, a type of natural evil.
This, though, is not the only type of evil that befalls man. All creatures may suffer evil according to some defect in their generation, but man can suffer an evil according to his will, which is another principle of motion. Natural generation seeks form as its good and will seeks good as its end. In seeking the good as its end, the will also seeks the means to reach its end, the process of which is called voluntary action. In creatures whose wills can reach and end other than their appointed one a deficiency in voluntary action may occur. This deficiency is the evil of which people most commonly speak.
The will of the creature always seeks the good of the creature. Insofar as the will seeks the perfection of the willing creature, the will cannot deflect from such an end. The perfection of creatures', though, consists in God's goodness, which is a good that is external to the creature itself. Because of this, the evil of will is possible. In other words, because God creates creatures that have their own wills and intellects, there exists a possibility that these creatures (angels and men) will choose to persist in what they consider their own personal goodness instead of that goodness which is the end of all things, namely, God's goodness. This is a possibility that arises from the very nature of created intellectual beings.
"This act of self will on the part of the creature, which constitutes an utter falseness to its true creaturely position, is the only sin that can be conceived as the Fall. For the difficulty of the first sin is that it must be very heinous, or its consequences would not be so terrible, and yet it must be something which a being free from the temptations of fallen man could conceivably have committed. The turning from God to self fulfills both conditions. It is a sin possible even to Paradisal man, because the mere existence of a self -- the mere fact that we call it 'me' -- includes, from the first, the danger of self-idolatry. Since I am I, I must make an act of self-surrender, however small or however easy, in living to God rather than to myself. This is, if you like, the 'weak spot' in the very nature of creation, the risk which god apparently thinks worth taking. But the sin was very heinous, because the self which Paradisal man had to surrender contained no natural recalcitrancy to being surrendered."
-C.S. Lewis, The Problem of Pain, pg. 76