At the things the Mara had done, what it had wanted to achieve, the Doctor had been there before. Bearing witness to what his own hands had done and fighting for his own mind to bring a stop to it. It wasn't him, but does the Doctor feel culpable, guilty? Yes. He would go to any lengths possible to stop what evil the Mara had wanted, and yet he had been stuck, hoping that someone would be able to stop it or that the castle would end what was happening in time. He'd been beginning to lose hope.

He hears Will's voice on that other level but doesn't respond in kind. Right now it's too much energy than he has reserves left for.

"You could have done that?" It's a hard realisation. Chances are, Will would have burned out more than just the Mara. "I'm so sorry. You had to make a choice, Will. There's nothing wrong with making a choice, no matter how unfair, and..." It was bad enough what the Mara had done already. Hurting more people, or God forbid, ending them in his form and name, it was everything he found abhorrent and stood against.

"You tried to save me and I owe you a debt of gratitude so great. But if I ever pose a threat, if I become a danger to those around me and whom I care about, if you can finish it Will, you must. These are the choices we must make." His words are careful but the implication is there; Will should have done it. It's a choice he's had to make before as a Time Lord.