As part of the universe of opposites, Lehi introduces an angel who is in opposition to God. The descriptions of this angel as the devil probably are due to the language of Christianity with which Joseph Smith was familiar, but the idea of a Satan as a figure in opposition to God was part of Israelite understanding. Satan was a heavenly accuser, best seen in the prologue to the story of Job. The fall from heaven symbolizes opposition to God, being outside of God’s influence and, therefore, purposes.
What is it that Satan the accuser sought that was evil before God? In this case, evil is the opposite of good, hence Satan seeks evil, in that he is one of the poles of the opposing pair. When Satan falls from heaven he became miserable forever, for he was outside of God’s plan that resolved the opposition between good and evil. The misery that Satan sought was to place humankind outside of God’s influence. Hence, when Satan tempts Adam and Eve to partake of the fruit of the Tree of Knowledge of Good and Evil, the impetus was to create sin.
When Satan promised that Eve would not die, but be as God, it was only partially true. The eating of the fruit, and the knowing of the difference between good and evil, was required. The only way that that would be effective was if they would not ultimately die, so what Satan promised was true, but it was not a truth that Satan could produce. It was a truth available only through the redeeming Messiah.