It started in primary school.
Jim could only tolerate the other children for so long before he just shut them out. They screamed and taunted. Their minds were dull, but their words were sharp. Tuning them out worked well, for a while.
He was still just a kid. A lonely little kid. But one with a brilliant mind. And from that mind spawned Professor Moriarity. He was a little man in a suit that sat on Jim's shoulder and kept him company. Finally he had someone to talk to! The little man made jokes with Jim. They sat in the corner at recess, having fun at exchanging scathing comments about the other children. Every now and then, the Professor would help Jim with a particularly difficult to remember problem on a test. It was like having two minds to remember things with!
At first his parents thought it was cute. Their child finally did something normal! They encouraged him. They didn't see the harm. Jim began talking only to the Professor. Occasionally he'd deign to grace his teacher with a response to her worried questions, keeping his words as blunt and concise as possible. She sent letters home, but the parents were to busy with the beginnings of the end of their marriage to look too closely.
In the months surrounding the divorce, Jim drew further into himself. Now even his teacher couldn't get a word out of him. The little Professor and him didn't even need words now, and so he never spoke at all. Why did he need to speak when his parents yelled enough for the whole family? Finally, his parents noticed.
"Jimmy, you're being awfully quiet."
The small boy just looked up at his mother. She shifted awkwardly, unused to the piercing gaze of those black holes her son had for eyes. Hadn't they once been soft and innocent? Hadn't he reminded her of a puppy only a few months ago?
She worked at it for a while, and after two weeks of failing to get her son to speak, she sent him to the psychiatrist.
Jim did NOT like the psychiatrist. Dr. Powers was a large, intimidating man. He asked questions, kindly, at first. When Jim stayed silent, the man turned to yelling. The tiny professor on Jim's shoulder told him things about the doctor.
"He's mad at you. You don't speak, and he doesn't like that. But it's not just that. The women in his office are his wife and his mistress. He brought them here to keep an eye on them, but it's backfired. His wife is starting to suspect the other woman and him of being together. He'd be nicer to you if you did what he wanted you to do. Speak to him."
And so Professor Moriarity taught Jim Brooks how to manipulate. He taught him how to get what he wanted.
Soon after, he stopped his weekly sessions with Dr. Powers. A little while after that, he stopped seeing his mom around the house. His papa started drinking a lot more. The Professor told Jim why, and so Jim didn't ask questions. He kept quiet and let himself be forgotten. And so Moriarity taught Jim how to keep to himself. He taught him how to be forgettable.
Jim stayed perfectly forgettable until 7th grade. Then he met Carl Powers. Carl was not a nice boy. Which would make sense, since he took after his father.
"I remember you! You were one of my dad's psycho clients! You never talked, just gave him that creepy look of yours!" Jim was noticed. And Carl continued to notice him everyday, and other kids started to notice him, too. The little Professor got angry at Carl, while Jim just got scared. The Professor started whispering things to Jim.
"See his arm? He's yet to put medicine on it. Think of how simple it would be to put some of that poison from your dad's office in the cream. Or just put it on his sneakers. No one would ever know. You could fade into the background again. Or better yet, you could fade into the shadows. Become KING of the shadows. No one would notice you, but every one would fear you. Brilliant plan, eh?"
The tiny man had never led Jim astray, so he did just as he told him. And just like every other plan the Professor had, it worked perfectly. And so Moriarity taught Jim to kill. He taught him to plot, and he taught him to rule.
—
He looked at Sherlock. Sherlock was the Professor's favorite. He liked Sherlock more than he liked Jim. Now Sherlock was going to die, and there was nothing he could do about it. The Professor was snickering as he sat on Jim's shoulder.
Jim was tired. He was tired of playing this game. He was tired of being King in the shadows. None of it was fun. All of it was lonely. Suddenly, Jim had an idea. Jim knew how to end this game. He knew how to make it all go away. How to make the Professor go away.
He was only half listening to Sherlock. He only ever half-listened to anything. Moriarity always told him what to say. He got close to Sherlock, and grabbed his hand.
"Thank you. Bless you." for finally giving Jim a way out. The little man on his shoulder shifted uneasily. He didn't quite know what was going on. Jim quickly grabbed the gun from his waistband and put it in his mouth. The last thing he saw was Sherlock's stupefied face. The last thing he heard was the shrill scream coming from his shoulder. And so Jim put some of the lessons Moriarity had taught him to work.