What Do You Mean, You Raised a Righteous Saint to Be Crooked?

"She was originally a witch. How could she have learned bad things because she met a demon cultivator like me? Those critics always say some ridiculous things." In 2026, the Era of Immortal Cultivation, the demon genius Lu Yu was interviewed by reporters. When asked about his past with the once righteous saint and the recent signs of the growth of demonic power, he said: "Although I am a member of the Demonic Path, I have sacrificed too much to cultivate the Righteous Saint, and I have a clear conscience! Although her current approach is a bit extreme, she will definitely become a mainstay in the Righteous Path in the future!" … In the year 2030 of the Cultivation Era, Su Yuqing, the former righteous saint, unified the ten major demon sects and directly overthrew the five righteous sects. [Based on the above materials, please point out the culprit that caused Su Yuqing to become a witch, and tell why it was Lu Yu!