"Being your friend and being a lying, murdering bastard are not mutually exclusive," Terrence said, almost amused. "At least, they weren't."
Yes, it was coming back to him now, being able to laugh in the face of something that was terrible. Was he certifiably insane now? If he'd bothered to listen to any of the staff at St. Mungo's, yes, they probably would have told him so. But he would take this chaos, this destruction of everything, this desire to bring the world down around his own ears no matter what the cost, was so, so much better than ignorant bliss.
And if she hated him now that she knew who he really was, fine. He'd expected that, gauged it a price worth paying. Maybe she'd figure it out in the end, maybe not. Either way, he'd given her the freedom of will and mind that she would need to defend herself, even if he'd shattered everything else in the process.
He stepped back, shaking his head. He would leave, as she requested; he had no last words for her, felt no need to explain himself any further, or to let her know that he would be around if she ever figured out his intentions enough to feel comfortable contacting him again. If he hadn't misjudged her, she could take care of herself from here on out, and he wasn't going to give up on the possibility that they might be friends again, even if it wasn't until after the war was over. Assuming they both survived, of course.
"Take good care of her, Quaffle," he said, directing this statement toward the dog. Then he turned on the spot and Apparated away, leaving Katie to figure the rest out for herself.