Why Did the Immortal Lord Turn Dark?

"The current Sovereign is an unquestionable tyrant." "He crushes bones for their marrow and spills blood to feed the starving, driving every immortal sect to the brink of ruin." "He burns the immortal path and abolishes the mystic sects. His selfish, despotic heart chills everyone to the bone." At first, when Chi Lu heard these claims, he thought they were nothing but nonsense. The Sovereign he knew was a wise ruler who worked tirelessly day and night and attended to every matter personally. He was the Divine King who led the immortals and celestial clans against countless demons and evil spirits, as well as a kind soul who had shown young Chi Lu great affection. So Chi Lu assumed that the immortal sects had made up these rumors out of hatred and jealousy toward the Sovereign. They couldn't possibly be true. That confidence lasted until he was thrown into Mount Tian... From his high seat, Lord Jiuyuan looked down at Chi Lu. The candlelight flickered over the dense, intricate black patterns on his robes. The Sovereign, once gentle and composed, wore a dark expression and a cold gaze. His words were just as chilling: "Take him away. Lock him in the dungeon." Chi Lu: "...?" And so the bluebird raised by the Sovereign knelt beneath the jade steps, plunged for the first time into deep self-doubt. By a twist of fate, Chi Lu found an unmarked, melodramatic xianxia novel in the dungeon. He discovered that most of its details were the exact opposite of what he knew. In the book, for example, the Sovereign had become a brooding, obsessive tyrant, and the other immortals and gods were completely unlike the people he remembered. Only their names remained unchanged, reminding him that this was the world he had lived in for hundreds of years. Though deeply troubled, Chi Lu still couldn't help feeling curious: So why did the Sovereign turn evil? Please note: 1. The love interest has had other partners, while the protagonist is inexperienced. The protagonist also has a difficult past, so read with caution if this bothers you. 2. This is a freewheeling creation. The logic is dead. 3. I can't think of anything else, but I insist on having three points. Theme: Face difficulties without fear, and rise to meet them