Alexander Luthor, Jr. was born in the alternate reality known as Earth-Three, the son of a different Lex Luthor & Lois Lane-Luthor. During case “Crisis on Infinite Earths,” the Anti-Monitor destroyed Earth-Three, along with multiple other realities. But Alexander’s parents sent him to an abandoned satellite on Earth-One, the standard Earth most things took place in. There, he ages rapidly, growing into an adult to help defeat the Anti-Monitor.
Later, a police officer asked if Luthor is ever tired of losing to Superman. Luthor said he would consider it a victory if he caused even one person to hate Superman. Cut to Mike’s brother Henry burning his Superman collection, having lost faith in him forever. Luthor wins again. Taking away a child’s faith in Superman may seem like a small thing, but the calculation and callousness involved makes it one of the worst acts Luthor ever committed.