Then they said to the eye: 'Do thou sing out for us.' 'Yes,' said the eye, and sang. Whatever delight there is in the eye, that he obtained for the Devas by singing; but that he saw well, that was for himself. The Asuras knew: 'Verily, through this singer they will overcome us.' They therefore rushed at the singer, and pierced him with evil. That evil which consists in seeing what is bad, that is that evil.

In plain words: Then the gods asked the eye to sing. The eye kept some of the pleasure of seeing for itself. So the demons struck it with evil, and that is why we can see bad things.