In the premortal council, Satan said he wanted everyone to be saved. What was so wrong with that?

In the Grand Council, “Satan … came before [the Father], saying—Behold, here am I, send me, I will be thy son, and I will redeem all mankind, that one soul shall not be lost, and surely I will do it; wherefore give me thine honor” (Moses 4:1). It may seem like Satan had noble intentions, but he didn’t. He wasn’t so much concerned with saving everyone as he was with taking the Father’s honor, which is the Father’s power (see D&C 29:36). “The devil … sought to take the kingdom of our God and his Christ” (D&C 76:28). He “rebelled against [God], and sought to destroy the agency of man” (Moses 4:3). “He became … the father of all lies” (Moses 4:4).

Satan’s uprising was built on a lie. He said he could save everyone, but he also would have destroyed agency—our ability to choose and act for ourselves—by taking it away or by removing consequences. Heavenly Father’s plan for our eternal progress requires agency—choices and consequences. His plan also requires a Savior to atone for our sins and make repentance possible. So Satan’s offer was deceptive nonsense. All he really wanted was power. And he was willing to tell any lie to get it.