The first time Mary and John met was nearly the last.
It was over two years before she would see him again, when masked, she strapped him in a vest full of bombs. He managed to get a laugh out of her with his sarcastic thank you, although she didn't let is show. Briefly, she felt a twinge of guilt for what she was doing. It wasn't the first time. Assassins, she found, typically divided themselves into three groups. She fell into one of the largest ones- the group of people who weren't properly detached, who still occasionally felt what they were doing and only stayed in because getting out was difficult and they had few other choices. The second, the lifers, were the only ones who were properly numb. They had somehow survived years and years of the job without being killed, jailed, compromised, or driven out, and they were few and far between. Mary had only met one and heard of two others. Then again, most of them had to be in such deep cover that there could be millions for all she knew. No one wanted to be a lifer.
The third type of assassins, the one Mary and all the other guilty assassins stayed away from, were those that enjoyed it. They were a different kind of numb from the lifers- they found it all fun, pulling the trigger and watching someone's head turn into liquid because of them. Honestly, Mary thought they filled her and the other reluctant assassins with such disdain because they knew they were like them, deep down inside. If they took no pleasure from the job, they wouldn't be doing it in the first place- you had to be good at killing to be more than a one-time assassin, you couldn't just be indebted to someone, although those kind of jobs had taken over Mary's life lately. She did enjoy sometimes, men dying at a slight movement from her finger. Only three muscles and a black mask to change the course of history. She enjoyed it more at first, when she worked in the CIA and felt sure she was only killing the bad guys. That was before she started thinking too hard about things, before they fired her for it and she ended up sinking deeper and deeper owing people a job just to stay alive. That wasn't to say she didn't still get a pervered sort of pleasure from time to time. Just not the day she met John.
John Watson was definitely a good man, Mary knew it as she coerced him into a stall and took her place in the balcony. Her impression was confirmed when his friend arrived (Sherlock Holmes- Mary knew he was special because Moriarty didn't insult his intelligence on the way over) and John Watson, in the ensuing confrontation, tackled Moriarty to let Holmes escape. Tackled the most dangerous man in England, in the world perhaps, even though it would definitely get him killed. So although John wasn't the first target Mary felt guilty about, he was the first who truly shook her to her core. When the job was done, she left the radio off on the ride back home. I will find a way to get out, she thought, no matter what it takes.
It took her years to set it up- she bought blonde hair dye, saved up money for a bit of plastic surgery and a lot of makeup, and came up with a new name- Mary, like the innocent virgin mother. It was deliciously ironic, and anyways, she'd always liked the sound of it. Unfortunately, along the way, she got help with her fake corpse from a man who later went into journalism and changed his name to Charles Augustus Magnusson, but at the time, he seemed well, about as trustworthy as anyone else in the business when Mary is that desperate. She also followed John Watson's career, and his blog, and by the time Hatman had jumped off the root and the world no longer believed in Moriarty, Mary had disappeared on a job, presumed dead, and was finally, finally left alone.
She wasn't sure what possessed her to become John's assistant. It was a bad, bad idea- John might have recognized her, or Moriarty might have tried to kill John and found her, or one of her older bosses could have tried to take a shot. She followed him around one day in disguise, unable to resist her curiosity about the stranger who changed her life, and that was the day Mary decided to apply for the job. John was so visibly lonely and crushed that she felt obligated to befriend him somehow. He saved her life, so she would try to save his. She faked a resume and recommendations (her previous job had made her surprisingly good at forgery) and was unsurprised when she got the job. The romance, however, she was not expecting. She even tried to avoid it at first. She did not feel worthy of John's goodness and loyalty after what she'd done to him and others. Eventually, she had no choice but to give in and hope he never found out.
She lay awake one night, reflecting on all of this with one hand over her pregnant belly. It was another one of those moments when she felt she had to tell him- their first date, their third date, the first time they slept together, their "I love yous" and moving in and engagement and marriage- all of the normal milestones of of a relationship filled her with anxiety and guilt. She should tell him, before things got any further. Before they have the child, or before the child is old enough to understand what she and John will be fighting about, or after the child left the house, or the 10th anniversary or the 5th or the 50th...but she couldn't, she was too afraid of losing the sleeping man next to her to say anything. John Watson was the best man she'd ever known, and she didn't want to hurt him or betray him or lose him with her secrets. He made her happy, and she thought she made happy as well, which was all that she needed. She and John brought each other back to life, and she'll do anything to keep him that way.