Marionette

Harry overhears that he's a horcrux and that Dumbledore knew about his abuse all along. When Vernon gives him the worst beating of his life, he apparates to Malfoy Manor. He knew Voldemort was there but he was going to die anyway if he continued as a slave to the light. He takes control of his own fate and offers his allegiance to the dark lord. Prophecies aren't law, after all.