Sebastian Moran and Jim Moriarty were deeply in love, for at at least six or seven years now, maybe much longer. But, their relationship was painful. Literally. Sebastian was cruel, but he could be nice when he wanted to be. No matter what happened, though, Jim still loved him, no matter what came his way or how many cuts and bruises he had. He wore them proudly, as if they symbolized their love.
Then, finally, Sebastian finally figured out that he was wrong. His perspectives on his mean actions were much different, and he realized he couldn't go back and fix them. He had hurt Jim, and that was enough to break him. Which it did. The guilt became too much, and he ended his own life. Jim, who was having a rather innocent day out of the house, came back to see the man he had fantasized a wedding and children with dead, laying with a blood-stained photo of them together, one of their favorites.
He mourned.
He ran.
He wept some more.
But the thing was, he had found Sherlock. And he had changed him. The previously sarcastic, mean, emotionless consulting detective had switched personalities as soon as Jim stepped into the door of 221B. He had gone from careless to nice and comforting, with the snap of not his fingers, but Jim's heart.
Overview: Betrayal
John has been compromised, corrupted, by consulting criminal. Sherlock was devastated, to say the least Sherlock spirals down into darkness, the only source of comfort at the tips of needles and razor blades. He was willing to make a deal with the devil.
Redemption comes in the form of the most unlikely source: an old enemy. Sherlock forms an uneasy alliance with the second most dangerous man in London. Sherlock makes a deal with the devil. Moriarty had stolen his best friend, it was time for him to return the favor for payback, and the gloves come off. The line between friend and foe blurs.