A/N: Major Trigger Warnings in this chapter. If you cannot handle gore, please don't read. I was unsure of what they meant by 'true monster', so I went in this direction. Seriously, insanity and murder (Minor character death)

Day #6: True Monster

Do Unto Others

(Rating M)

He was going to ask Mary to marry him. He had mentioned his intentions to Sherlock, and the man had been less-than-thrilled, not that John had expected anything more out of him. He had told him on multiple occasions that love was a weakness, and that marriage was folly, but John couldn't be half-arsed to care. Mary was beautiful, funny, and he was completely in love with her.

He planned to ask her while they were at dinner. She would meet him at 221B after work, and then John would mention that he had made reservations at a posh new restaurant nearby. They would have a good time, and during dessert, he would get down on one knee and pop the question. Really, it was fool proof.

Work lasted far too long, and John was a bundle of nerves, jumping each time his mobile chimed, even though he knew a majority of those texts were from Sherlock complaining about his boredom. When it was finally time for him to leave, John moved quickly, pulling on his coat and bounding for the door, sucking in a deep breath before starting on his way home. The day was beautiful, but he couldn't even begin to enjoy it. His fool proof proposal didn't seem so fool proof anymore, and he kept imagining the million ways that Mary could say no. By the time he arrived at Baker Street, John was considering postponing the proposal, at least until he was positive that she would say yes.

John ambled up the staircase, entering the lounge with a loud sigh.

"Ah, John." A deep baritone voice said, making him jump. John looked up at Sherlock, a wry grin on his face. Sherlock was wearing his purple shirt, one of John's favourites. In one hand he held a flannel, looking as if he had been busy cleaning up, though knowing Sherlock, he was probably testing the flammability of the microfibers.

"Hey, Sherlock." John replied, rubbing a hand across his forehead. "How are you doing?"

Sherlock didn't answer his question, choosing instead to study him, looking disappointed with what he saw.

"You're still choosing to go through with it."

"Pardon me?" John frowned in confusion. Sherlock merely sighed in response, moving towards John quicker than he thought possible. The brunet wrapped his left arm around his middle, twisting him so John's back was against his chest. His other arm came up, pressing the flannel against his mouth and nose. John gasped in shock, horrified to discover that the rag had been soaked in chloroform. John began to struggle against the vice-like hold, but Sherlock had the upper hand. Far too soon the chemical began to take hold, blackness swimming across his vision. Before he blacked out, he could feel himself being gently lowered to the ground, Sherlock murmuring in his ear.

"Shame. I wanted to spare you from this..."

When John regained consciousness, his head was pounding. He groaned, trying to get more comfortable in the hard wooden chair. As he shifted, however, he discovered that he was bound tightly, legs and arms unable to move.

"Good, you're awake." Sherlock's voice broke through his scattered thoughts. John opened his eyes slowly, not wanting to see what was in store for him. When he saw what Sherlock cooked up, he wished he had kept his eyes closed. Mary sat in front of him, bound in the same manner as John. The only difference was that she had a gag in, whereas he did not.

"What in the bloody hell is going on, Sherlock?" John asked, watching the terror on Mary's face.

"I told you that you shouldn't do this." Sherlock's voice came from behind him, and John was unable to turn his head to look at his flatmate. "But you didn't listen."

"What are you on about?" John snapped. Sherlock walked around the chair, twirling a small knife in his fingers.

"You. Getting married. It's ridiculous. You belong to me."

"No I don't, Sherlock." John replied. He should have thought his answer through, but he didn't believe Sherlock was capable of what he did next.

Sherlock gave John another disappointed look before burying the knife in Mary's arm. The gag muffled her scream, but John yelled loud enough for the both of them.

"What the bloody, badgering fuck?!" John shouted, staring at the bloody wound.

"Such language." Sherlock scolded, tugging the knife from Mary's arm. "This is punishment. You're being punished for thinking you're allowed to leave; her, for thinking she can take you."

"She wouldn't be 'taking' me." John growled, watching as the other man trailed the sharp point up Mary's arm, leaving a trail of now-drying blood. "We would be getting married, and you and I would remain friends."

"Unacceptable." Sherlock cut John off, digging the knife into Mary's collar bone, carving out a nonsense pattern.

"Stop this!" John growled. "Stop this right now!"

"Do you still plan on asking Mary to marry you?" Sherlock asked, pulling the knife away from what looked like a bloody mass of swirls.

"Of course."

"Then no." Sherlock sighed, putting down the small knife, grabbing a large kitchen knife instead.

"Sherlock, no-"

But Sherlock had already moved, slicing Mary's left ring finger off in one quick slice.

"Oh god." John groaned, unable to look away from the blood gushing from the trembling finger. "Sherlock, please stop this!"

"Not until you choose me." Sherlock's voice was calm as he took the knife and carved out a hunk of skin from Mary's back, perfectly preserving a small tattoo. He slapped the hunk of skin down on the table in front of him, giving it little more than a cursory glance. "It's not as if you have a choice, John. Really, all you are doing is prolonging her suffering."

"Please make it stop." John begged, tears beginning to roll down his face.

"Say it, John." Sherlock sang, walking behind Mary, his knife slicing a line as he went. "Say it!"

"You!" John gasped. "I choose you. Please stop hurting her!"

"If you insist." Sherlock said, shrugging before he slashed Mary's throat, cutting neatly through her carotid artery. Blood gushed from the cut, pouring down the woman's neck as she died.

"Mary!" John cried, struggling against his bonds. "You killed her! You killed her!"

"Of course I did." Sherlock replied. "You told me to stop hurting her. She will never be hurt again."

"You're a monster." John shook his head, struggling even as the ropes made his wrists bleed. "You killed her, and you don't regret it at all."

"Of course I don't." Sherlock said, walking over to John. He leaned over the man, brushing his blood spattered hand against the blond's cheek. "Now you're mine forever."