Because Lucifer “had fallen from heaven, and had become miserable forever, he sought also the misery of all mankind… . He seeketh that all men might be miserable like unto himself.” And what is his misery? That he can never be with God? No, he hates God. He has already decided that he does not want to be with his Father and his Brother, the Savior. His misery is that he can never have a physical body, never be a husband, and never have children or posterity. He particularly wants us to abuse the sacred procreative powers so that we will be denied those powers in the next life and live singly forever—and be miserable as he is.
Lucifer became a devil, which word derives from Greek diabolos, meaning “slanderer”; and he became a satan, which is a Hebrew word meaning “adversary.” He took on other descriptive name-titles, such as “serpent,” “father of lies,” and many others throughout scripture, including the evil one, dragon, perdition (“lost one”) and Beelzebub (literally, “lord of the flies”).
Satan specializes in subtle half-truths, such as “partake of the forbidden fruit, and ye shall not die”—which was a lie—“but ye shall be as God, knowing good and evil”—which would become the truth.