"Luna Lovegood, " Theodore said, after swallowing the lump in his throat; all the same, his voice cracked on the name. "I would have tried to heal her, too, but she was dead before I got to her side. She didn't order me to do anything - she just asked, and I... I couldn't say no."
That was the lie, the only lie, but there was a lot of truth to it, as well. Luna had never asked him to do anything, but her sheer presence, the way she'd looked while she was being tortured, had essentially done precisely that. He truly believed that she would have wanted him to help Neville, and wanted him to spare Hermione by giving her name instead, when there was nothing that Rosier could do to Luna now. And yet, he felt genuinely guilty for betraying her, all the same.
And then, the truth. He wasn't trying to appeal to any sympathies Rosier might have; he wasn't even sure Rosier had those. He just wanted to make sure that his true intentions were very clear. "I never wanted to hurt anyone, sir, I've only ever wanted to help people. And I'm sorry, I really am. I'll never do it - or anything like it - again."
He meant all of it with every fiber of his being. All he wanted was to help, to heal, to find a way to bring some semblance of peace to the crazy world he'd had to live in. And through his terror, he was finding that, a little bit, by being caught. Theodore wasn't made for sneaking around, never had been; he much preferred the straightforward path. He wanted to be the honest person he had always been, and this was his chance to make that right.
The only thing that might have changed his mind was his punishment. He didn't know what was in store for him now, and he wanted to believe that he would be strong enough to accept whatever it was, but he had no idea what to expect from Rosier.