"Goodness is, so to speak, itself: badness is only spoiled goodness. And there must be something good first before it can be spoiled.... It follows that this Bad Power...must have things to want and then to pursue in the wrong way: he must have impulses which were originally good in order to be able to pervert them.... To be bad, he must exist and have intelligence and will. But existence, intelligence and will are in themselves good. Therefore he must be getting them from the Good Power: even to be bad he must borrow or steal from his opponent. And do you now begin to see why Christianity has always said the devil is a fallen angel? That is not a mere story for the children. It is a real recognition of the fact that evil is a parasite, not an original thing. The powers which enable evil to carry on are powers given it by goodness. All the things which enable a bad man to be effectively bad are in themselves good things - resolution, cleverness, good looks, existence itself.... Christianity thinks this Dark Power was created by God, and was good when he was created, and went wrong. Christianity agrees with Dualism that this universe is at war. But it does not think this is a war between independent powers. It thinks it is a civil war, a rebellion, and that we are living in a part of the universe occupied by the rebel." (Lewis, C.S., Mere Christianity. HarperCollins, 2001, pp.44-45).
There was a war in Heaven between Jesus Christ and Lucifer. Satan rebelled against God and God's plan of salvation. At one point, as C. S. Lewis pointed out, Lucifer was good. He had intelligence, light, knowledge, and free will. He had the moral agency to choose right from wrong. These are the good things Lucifer received from God. He took these gifts and used them to become evil. It is only in this manner that God created evil. There are eternal principles - God's laws - that are not breakable. Agency is one of them. God gave Lucifer - and all of us - agency, which agency Lucifer used for evil. Lucifer rebelled and was exiled to the earth. "Wherefore, because that Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, and also, that I should give unto him mine own power; by the power of mine Only Begotten, I caused that he should be cast down" (Moses 4:3).
With this agency we all have been given comes the responsibility to choose that which is right. "That every man may act in doctrine and principle pertaining to futurity, according to the moral agency which I have given unto him, that every man may be accountable for his own sins in the day of judgment." (D&C 101:78). With this agency - the same gift that made Lucifer's fall possible - we can choose good or evil. Do you choose good?