Tenecion777
When Harry has had enough chaos in life and decides to become it.
OR
Harry has had enough of the manipulation, enough of the burdens that come from the ministry giving him auror work to do, and he's had enough of people stabbing him in the back. So, like Voldemort before him, he snaps, but he's always been different, how kind that, and is given help by an infuriating yet enjoyable presence.
PS: Hadrian is an absolute little shit, and he wants to make your life as difficult as possible by being as annoying as possible without pushing the line too far.