It didn't happen all at once; it rarely does. Not one single moment that flipped the switch from good to evil. It probably wasn't even the abuse that started it — it was the neglect. A decade of singular self-reliance leaves its mark. The abuse started the anger, the simmering fire that never quite banked. A bigger factor was the constant manipulation, the perception that he was a tool to be used to achieve some grander purpose. Not a child, barely even human in their eyes, simply a thing to be wielded. It was the adults, the supposed role models, the ones he should have been able to rely on, that failed him most of all. If pressed, if one was forced to answer the question of what turned Harry Potter evil, that was it. The utter failure on the part of each and every authority figure in his life to make even one correct decision, to put the boy ahead of the cause, ahead of their past, ahead of their own guilt. Once that final piece fell into place the fire grew into a raging inferno, spilling over the bounds of flesh and propriety, burning the bonds of friendship and family, razing the very structures that held him. There were no survivors, not even Harry himself.