Why did God create us when He already knew who would enter Paradise and who would enter Hellfire? And why did He send Iblis down to Earth, knowing that he would be among the inhabitants of Hellfire and that his repentance would not benefit him?

Allah created creation so that they would know Him, affirm His Oneness, and obey Him. The purpose of creating humankind is for them to know their Lord and worship Him alone. As for the ultimate goal intended for them, it is recompense with justice and grace, reward and punishment. Allah does not admit people into Paradise or Hell merely by His foreknowledge, but rather by the deeds they performed in this world, because Allah sent messengers and revealed books so that people would have no argument on the Day of Judgment. As for Iblis, he did not descend to Earth repentant, but rather defiant and arrogant, to be the leader of the party of Satan, and for Allah's wisdom in testing His creation to be fulfilled.