Totally just hitting the AUs here.

Official warning: Dark themes, murder, sexual shit, swearing

...

It took her until January of 1999 to find Greyback. In hindsight, she should have demanded more detailed answers from Dolohov, but it was too late to dwell on that. She listened for rumours about werewolves, and followed the rumours everywhere.

She found herself in Vatra Dornei in Romania, nestled in the Carpathian Mountains. She found Greyback there, and morphed herself once more, to get his attention. And it worked.

But this was where her plan went downhill. She had managed to get Greyback to drink with her, but as soon as they were alone, she realised she had made an error. Greyback was bigger than Dolohov, and the fact he allowed himself to be taken over by the wolf, it didn't help her in the slightest. The one thing that worked in her favour was that he had placed a Silencing Charm on the inn's room, meaning no one would hear a sound from within.

However, she gave herself to him, hoping to use it as a chance to catch him off guard. As disgusting as it was, and how she knew it should be Remus she was with- she would cry most of the next day, while she tried to heal the claw marks on her back and neck and face.

He was animalistic in a way Remus never was. Where Remus was gentle, caring and slow- Greyback was rough, painful and demanding. Remus allowed her total control, Greyback simply took total control. He had spent part of the time, with her bent low on her knees, and the last couple minutes, forcing her to look him straight in the eye- taunting her, telling her to say his name and not Remus'. She said nothing, which further angered him and earned her the scar on her cheek.

And when he rolled off of her, falling onto his back, she rolled out of bed and grabbed her wand- another stark difference between the two werewolves, she wouldn't have gotten up for the rest of night with Remus next to her- and she did feel terrible that she was comparing the two men. She bound Greyback quickly before he had a chance to react.

"Where's my son, Greyback?" She demanded, aiming her wand at him.

"You bloody bitch-" Greyback growled.

She aimed her wand at his crotch. "I'll burn you." She threatened. "Where's my son?"

"What would I want with a half-werewolf?" Greyback demanded.

"Incendio!" She cried, and Greyback's screams fill the room. She non-verbally doused the flames with water. "Where is he?"

"I won't tell you a damned thing, you bitch!"

She pocketed her wand, and pulled out the pocket knife. "Very well." She said, shrugging, and walked over so she was beside him. She dragged the knife across his skin, as he tried to get out of the bonds, his need to escape becoming more desperate.

"Will you tell me now?" She asked, stepping back as he bled.

"Fuck you!"

She shrugged and leaned against the wall, pulling out her wand again. "Sectumsempra!"

Slashes appeared all over Greyback, blood soaking the sheets. "Fine, Rookwood! Rookwood's in Scotland, he'll know!" Greyback yelled. "Heal me, you psycho bitch-"

"Sorry, I don't know the counter curse." Tonks said, and she simply stood by, watching the blood drain from the werewolf before her. And once he was dead, she slipped from the room, not wanting to be caught there when someone found the body.

Auror Interrogation Room, 2001

"You killed Greyback?" Harry asked, staring at her, shocked. "You shagged Greyback, and then killed him?"

"I've hated myself ever since it happened, Harry." Tonks said. "Not the killing part, the other part. I was desperate, I wanted to find Teddy-"

"We- as in the Order- have had Teddy since July of that year." Harry said. "Why did you not figure out that the Death Eaters didn't have him?"

"I did." Tonks said. "After I spoke to Rookwood-"

"Rookwood's dead. Did you do that as well?"

Tonks shifted under Harry's gaze. "Yes." She finally admitted.

"There have been thirteen dead Death Eaters since the war. Are you going to tell me you killed thirteen people?"

"No." Tonks said quickly. "I only killed four of them, I have no answers for the rest."

"And Rowle, was he going to be the next-"

"Yes."

"But if you knew, after talking to Rookwood, that they didn't have Teddy anymore... why did you keep killing them?"

Tonks took a deep breath. "I had to, Harry."