What Do You Mean, You’ve Led the Righteous Saintess Astray?

“She was a witch to begin with. How could she have turned bad just because she met me, a demonic cultivator? Those critics are always saying ridiculous things.” In the year 2026 of the Cultivation Era, demonic path prodigy Lu Yu was being interviewed by a reporter. Asked about his past with the former Holy Maiden of the righteous path and the recent signs of the demonic factions’ growing power, he said: “Although I belong to the demonic path, I gave so much to cultivate that Holy Maiden of the righteous path. I have a clear conscience! Her methods may be a little extreme now, but she’ll definitely become a pillar of the righteous path one day!” … In 2030 of the Cultivation Era, Su Yuqing, the former Holy Maiden of the righteous path, united the Ten Great Demonic Sects and swept straight through the Five Righteous Sects. [Based on the information above, identify the culprit who caused Su Yuqing to become a witch, and explain why it was Lu Yu!]