He looks down at his trainers a moment, crossing his arms over his chest. "Davros."
He looks up again. "Davros came back. Stole a bunch of planets right out of the sky. Including this one. Whole new Dalek empire. And...he was planning on using this reality bomb...wipe out all existence. Which would include you, too, by the way. He would have just kept trying, I had no choice..." his eyes are sad, but his face quickly hardens. "No, I did what needed to be done."