Sherlock Holmes always had trouble following the objective of the angels.
He'd died in 1880, following a mishap in the laboratory. No one knew, as he'd died working alone. At first, he himself didn't even notice. He was too busy with his experiment, the noxious fumes smothering him and bringing him to the floor before he had a chance to open the windows. All he knew was that he'd lost consciousness in the middle of working, which to be honest, wasn't that uncommon. Something in his head clicked, and his already fervent passion for the law was given new kindling. He didn't register that he was no longer alive until he went to open the window and realized he hadn't been breathing for at least three hours. "How peculiar," was all the thought he gave it. If he was alive, he was alive, and it didn't matter the why or the how.
He'd found out the larger objective the day before he met John Watson a year later. It had come in the dreams he no longer had any purpose for but nonetheless forced on himself using cocaine. There were no words in the communiqué, just a series of images and sensations. He dismissed it as a hallucination, but thought it a very curious thing indeed. (It was some time before he accepted it for what it was, instructions from Above, and even then was reluctant to obey.)
By the time he and Watson had grown close, Sherlock had learned to coerce his angelic body's heart to beat, the breath to flow, the digestive system to work. He'd used his uncanny abilities in the world of disguise to make some attempt at aging. He could continue his work without anyone suspecting a thing. He only shed the layers of rubber and makeup once the first John Watson he knew died.
It was twenty years before he found another friendship. His brother had long since joined him in the Beyond, and Mycroft had been personally charged with keeping Sherlock on-task. During the gap between the Wars, Mycroft was the only person he could talk to, the only person he felt he could trust. He neither wanted nor felt he needed anyone else. He wanted to work alone. The second John Watson entered his life, no direct relation to the first, though it was a common enough name. This Watson was a bit bungling, occasionally moronic, but it made Sherlock feel alive again to have his company, and he formed a bond with him as he had with the previous John Watson.
But time is a cruel mistress and even he was snatched away, unlike Sherlock, never to become an angel but to rest, his work in the mortal plane done.
The other angels wanted Sherlock to keep some semblance of order in his life. They wanted him grounded. Sherlock had long suspected that it was why the two Watsons had been brought to his afterlife, one at the start of his journey, and another when he'd begun to drift away from his pursuit of justice—they were his anchors. His fiercely keen brain was still needed to fight the evils of Man which they inflicted upon one another time and time again.
Time passed. The century turned. Sherlock Holmes stood in the laboratory of St. Bartholomew's Hospital, tinkering with chemistry yet again. Angels get bored, particularly this one, and there were too many nights to count that he'd turned back to cocaine, though it was getting harder and harder to find because of the laws. He'd run out only the previous night as his mind screamed for something to do. Now he was running unnecessary chemical tests just out of a need to keep himself occupied, the world's only consulting detective fighting a screaming depression and hiding it well. Then came the third John Watson. This one was stronger than the last. Like the first, a war victim. Like the first, teetering on the brink of his own personal Hell. Part of Sherlock wanted to scream at whoever had sent this Watson his way, tired of being used to look after Watsons that didn't really need looking after. Part of him could tell he was going to form the tightest friendship yet.
Contrary to popular belief, angels have no wings. They look much the same as the rest of us, and there's nothing to slow a fall. He'd fallen so many times, literally, from waterfalls and buildings and bridges and now he was falling from Bart's. Moriarty had shot himself not five minutes earlier, which had shocked Sherlock not because of its suddenness—Sherlock knew all-too-well the pain of being bored to death—but because he'd assumed that, like himself, Jim Moriarty was an Immortal. Certainly not an angel, though. But he'd been as mortal as everyone else he'd known and now there was so much blood on the roof.
He'd let himself fall. If he didn't, this newest John would have been killed, as would two other people he hadn't realized were his friends until that moment. John didn't know Sherlock was an angel. John didn't know he couldn't die again. Sherlock terminated the forced heartbeat and breath, let everyone think he'd died that afternoon, the injuries forcing his skin to open and blood to stream from his head and mouth, and he could hear John's half-dissolved plea to get to him.
"He's my friend." It was the first time one of his "deaths" had such an immediate impact, and he could feel the soldier's unmistakable hand on his wrist, checking for a pulse he didn't know had stopped over a century ago. It hurt—Sherlock knew how empty John had been when they'd met, knew that he'd filled a hole in his spirit, and he knew that in order to save his life, he'd had to rip that precious bond away.
The worst part for Sherlock was seeing John mourn, knowing that he didn't know why Sherlock had taken the fall. To John, Sherlock had killed himself because of a rumour that wasn't true and John had no idea of the conversation on the rooftop and couldn't figure out how the need to stop their adversary was followed by plunging to his death. Sherlock couldn't adopt invisibility—another incorrect stereotype—but did his best to watch John from afar, aching inside, but knowing that one day he'd return.