The Doctor always had a darkness in him, one that showed in every lifetime. It became more prominent in this one, though. In his 12th, with Clara by his side.
At fist Clara had been scared of him, she had gotten so use to the Giraffe like doctor, the one with sadness etched in his face. He could burn a world, and never forgive himself for. She loved that about him because it balanced her out. Clara had to work to be good, while that Doctor didn't. But now everything changed.
This Doctor was mad. He was like a Dalek. Always wanting to hurt something, enjoying their pain. And she loved this one even more. This Doctor was one she could relate to on terrifying levels. This was a Doctor who would take her out to kill off a planet, and find fun in it.
There are many stories about the man called The Doctor. Some call him a hero, others a monster. There is always an underlying tone that he can and will hurt others for something her believes in.
He showed up on her door step and held out his hand. No words were spoken, just a silent offer. One that she took.
Away they flew, through time and space, to a planet neither knew the name of. There were lives on that planet, families, homes, children, but neither cared. Neither Timelord nor human blinked an eye when they sat back and watched the planet die. Neither cared as they floated miles above, sitting on the edge of the TARDIS, and watched to murder and bloodshed they created, pull through.
That was how they were now, and it made them oddly happy. They hardly spoke, only acted, and that made them all the more real. This was a life they both wanted. One where they could kill, and never feel guilt.
The last time Clara had seen the Doctor, his crazy smile and wrinkled face, weren't the things she noticed about him. His eyes were what scared people constantly. His darkness no longer hid away, but showed how much he truly gave up. How much he truly gave in.