Adam and Eve were both told not to eat of the fruit of the Tree of Knowledge of Good and Evil. Neither of them had the slightest inkling to do so until the Serpent told them they had the option of disobeying God. Until they were aware they had such an option, they were unable to exercise their free will because there was nothing to choose between. Before the serpent presented them with a second choice, there was only one path to take. If there is only one path to take, then free will is meaningless. Therefore, the presentation of another option by the serpent was necessary in order to allow mankind to make use of the Divine gift of free will.
Certainly God him/herself could not present man with this choice-- had he/she done so, no matter what path one chose, one could say they were following the will of God. So God needed someone to act as the choice giver. Someone to point out to mankind the options available to him. Lucifer, whose name means giver of light, accepted this job. Lucifer "enlightened" mankind that they had the choice of acting selfishly or of acting out of Love. In fulfilling this task, Lucifer is simply fulfilling the Divine will of allowing mankind the freedom to choose to return to God and follow the path of Love rather than the path of selfishness.
Lucifer has been demonized, literally, by countless generations of humans. But just as the betrayal of Jesus by Judas Iscariot was necessary to the fulfillment of Jesus' "destiny", so is Lucifer's temptation of man necessary to the fulfillment of mankind's ultimate destiny. For without a choice, we have no free will. And with no free will, we cannot be held accountable for our actions. Without a choice, we are no more than puppets of the Divine Mind following a course of predestined actions. The acts of puppets are the responsibility of the puppeteer, not the puppet itself. Without Lucifer, there would be no "sin" because sin involves willfully violating the laws of God. Without an option, there is no choice so there is no sin.
Please don't get me wrong. I am not advocating worship of Lucifer in any way. To do so would be to willfully worship the selfish nature of mankind and in doing so only delay our return to the Divine Creator. But I am suggesting that we recognize Lucifer for what he really is: an angel doing a necessary, albeit unpleasant, task. It is my belief that when each and every one of us chooses to follow the path of Love, the Creator will turn to Lucifer, say, "Job well done, my child. Welcome home."