I honestly have no idea where this particular idea popped up from but...it now exists. There is no happiness or hope to be found here. Warnings for violence and blood.
Also I didn't actually describe the Doctor in here so it could be whichever incarnation feels right for you. I just had to pick one for archiving purposes but whichever Doctor works for you in the fic, tbh. I left it ambiguous on purpose :)
Title is from "400 Lux" by Lorde
Being an assassin was typically a solitary profession, the shadows and secrets and suspicion inherent with the title were not conducive to successful partnerships.
These days, though, you couldn't hire the Doctor and the Bad Wolf separately. They worked together like a well-oiled machine, slipping in and out of the shadows hand in hand, synchronized lies falling from their lips. The two of them were the best in the business and if you wanted someone dead and were willing to pay to make sure it was done right, they were who you hired.
John Smith and Rose Tyler had been in the business for years. They had started out with separate careers and had met when they were both hired by different parties to kill the same person.
The first time they kissed, their hands were covered in blood.
They met up all over the world. Weekends and stolen nights in Dubai and Moscow and Barcelona — gasps and sighs and wandering hands paired with faces in the crosshairs and slit throats and broken necks.
Killing together was more intimate than fucking.
It got around that they were working together and were more efficient than ever and it wasn't long before every job they took was a request for them both. There was more money to be made by being the novelty couple on top of their ruthless reputations.
They were so wrapped up in each other that they didn't notice when the shadows they walked in started to insinuate themselves into their hearts.
Assassins aren't known for having working moral compasses or a consciences, but when Rose and John had fallen into the profession they'd each had standards, limits about how far they were willing to go and who they were willing to kill.
If anyone had actually known the two of them, they would have seen the two assassins devolve. Would have seen the spark of empathy drain from their eyes, leaving them flat and dangerous. Would have seen them trapped in a downward spiral that was systematically stripping away their humanity and siphoning their souls away as they spilled a river of blood.
Slowly, they sacrificed themselves to their job. Rose Tyler and John Smith no longer existed but the Bad Wolf and the Doctor were alive and well.
They drained bottles of liquor in the aftermath of hits and fucked each other desperately, hoping to feel something, anything. Tried whatever they could think of to get the feeling of being alive back.
Nothing worked. There was nothing left to save, nothing left for them but the next kill, the next hit of adrenaline. There was nothing of two assassins left but shadows and shades of their former selves and a relationship that tied them together and was as stained with blood as their individual hands.