I Bullied the Fairy for Six Years Before Discovering She Was a Female Demon Lord

[Male Lead, Multiple Heroines, Heroines Pursue Him First]

The mission is to play the villain and seek out every suitable girl, then inflict pain and torment on them! The mission can only be completed once one of them is revealed to be the chosen heroine and transforms into a phoenix!

But can someone explain why the girl who seems most like the heroine, the little sister I've bullied for six years, is actually the book's biggest villain, the female Demon Venerable?

Now she's coming to kill me, along with all the other girls I've bullied!