His Horcrux. His Murderer.
Curses designed to kill, torture, and dominate other people. Harry had an obsession to research about the Unforgivable – being the sole proof that the Killing Curse is not unstoppable would do that naturally.
"You're a Dark wizard!"
Really. Harry did not mind being insulted. He did mind being insulted so poorly. So he does Dark magic. That doesn't earn him the label Dark. And if he does earn it-
The label is not something to be upset.