Disclaimer: I don't own Doctor who, except in my mind.
He spends weeks fighting with himself.
The TARDIS is in orbit around a star and he argues with his reflection in the mirror. It has become ugly, a grotesque thing that he cannot bear to see. Nevertheless, he forces himself to look, to see what he truly is.
Davros called him the Destroyer of Worlds. But that, he knows, isn't quite correct. A world is just rocks and sand and dust. It isn't home without people to live on it. He is not a Destroyer of Worlds, but a Destroyer of Lives. Precious, irreplaceable lives. So many have died in his name. He wonders if it will ever stop.
Oh, Donna Noble. I am so, so sorry.
He spends his days screaming at himself. He almost punches the mirror, the day after she's gone. It is easier to be angry at himself than at the Universe, something that he cannot truly grasp in his hand and crush into a million tiny shards.
The question hovers in the back of his mind, threatening to drag him under. Was it the right thing to do?
By all accounts but one, it was. Donna was going to die, he couldn't just let her. Wilf and Sylvia would have killed him, though that didn't matter in the grand scheme of things. He didn't fear their wrath, though he pretended to. That wasn't what mattered. Donna was safe. She would live a normal life, unharmed and protected.
The single account left, of course, was Donna's own. She would have rather died there in the TARDIS than go back to what she had been before. The splendor of the universe was trapped in her mind, never again to be free. The worst part was, Donna's own account equaled everyone else's. It wasn't because she was part Time Lord or because she was more important. It was because she was Donna. She had just as much right to decide her own life as anyone else had to decide theirs.
He had taken that decision from her to save her life, though she didn't want him to. The last thing she wanted was to leave, yet he had forced her to do just that against her will. It wasn't fair for either of them.
He looked in on her from time to time, just to make sure she was alright. He had watched her half a dozen times since that day. He always stayed a second too long and she had felt his presence in the back of her mind. Every time she turned to look and every time he ducked out of sight just in time.
He missed her. More importantly, he could see that she missed him. She wandered the streets and shops, never buying, only looking, with a far off gaze in her eyes. Those were the eyes of true, profound loss. His own eyes, peering back in the mirror, looked the same. Because of this, he knew that they had both lost their will to live. He sighed and turned away from her one final time.
He no longer had anything to lose.