Seeing Dumbledore in limbo, explaining how he was a Horcrux and that he could go back and fight if he wanted to, pissed Harry off immensely. He yearned to reunite with his family, yet it was still denied to him, so he settled for punching the old Headmaster in the nose. Harry might have become utterly ruthless and cold, but recognising that he had been set up for martyrdom and self-sacrifice didn't sit very well with him. However, he wasn't about to leave wizarding Britain in the clutches of Voldemort, no matter if he was mortal or not. Theoretically, he could end the snake bastard if he hit him with a curse by surprise.