Answer:
Step-by-step explanation:
He knows that evil in him is wrong. He also knows that it is quite likely natural. Use sex as an example. There are 613 laws in the Old Testament. I don't know how many deal with sex, but I would guess quite a few.
Augustine would know them all. Many are based on natural urges. He thinks his visions are foul, but that does not stop him from thinking about them. And if we take him at his word, he loved them.
The serving no purpose (continuing on with the example above), serves no purpose, because, though it is in his mind, he will likely not bring it into physical fulfillment.
Without reading more on these passages, this is about all I can offer you.