Before Harry and Miss Granger became betrothed, she was a "useful idiot," because of her slavish trust in authority figures; but now she must die for the Greater Good. After she dies, Harry will lose the will to fight me. After the Wizengamot forces Harry to return to Hogwarts, and with Miss Granger dead, I will hit Harry with a Confundus Charm or two, and he then will sign a betrothal contract with the Weasleys, which will serve the Greater Good.