It it known that the Prophets knew that the humans were truly descendants of the Forerunners, and that they also understood this before the other species of the Covenant. The Covenant, probably, were spying on the humans and were familiar with us before we were with them (by intercepting radio messages, long-rang telescopes, stealth-ships, etc.) So, the Prophets decided that, in order to keep their power, and obtain the Forerunner tech. on human planets, they had to destroy us and not let any other members of the Covenant in on their secret. If they did, the whole Covenant religion would be unmasked as a lie, and the Prophets would fall out of power, because the Covenant worshiped the Forerunners. Put the two together, and the humans are the children of the gods. Therefore, in conclusion, the whole, human-weak-evil-inpure thing was mostly propaganda created by Truth, Mercy, and Regret, and firmly believed by all other members of the Covenant, and the bulk of the society never really heard the truth of the humans until the Great Schism.