It all started with that first meeting, when Seb first got the job. That's how the guy introduced himself: "James Moriarty, consulting criminal." You could almost hear it in his voice: the only one in the world. I invented the job. So how was he to know, half drunk and bored out of his mind, that this was not James? James he would meet later. This man had never crossed paths with James, and if he had, he'd probably shot him in the face without a second's thought.
So he says "Alright James. You've got yourself a deal," and feels the atmosphere change and it's wrong, everything about that name is wrong for this man. He's a snake, the devil you run to with your problems, he is "Please, Jim, will you fix it for me?". He is not James. He hasn't been James since primary school. Hasn't been James since his Mum died at his Da's hands (but of course Sebastian doesn't know about that yet). So Sebastian backtracks and corrects himself; "Mr. Moriarty. Sir." And where did that come from? He hasn't addressed someone as 'sir' since he was discharged. Who the hell is this guy?
After a few days, it doesn't matter; he gives purpose to this dishonourably discharged soldier, gives him nicer guns than he's ever seen, fits him out with suits that cost more than an entire closet full of his clothes would have, before the military. He gives him jobs, gives him an apartment, gives him his choice of ammo. He doesn't give him a friend, but then who needs one? Seb's been fine for this long, he doesn't need anyone else. He gives him assignments, sometimes once a day, sometimes three in one hour, sometimes nothing for a week. He makes use of every talent Sebastian possesses, brings out his latent artistic ability, uses him to make statements. He's insane, but Seb doesn't mind. Crazy is something he knows; crazy, he can deal with.
Crazy isn't the best way to define Jim Moriarty, he realizes later. It's too simple, too neat, too common. The Boss is a tick mark in every box on a psych eval, everything wrong that can be, he's blood on your tongue and a knife in your hand and a cut that isn't allowed to heal. He's torture even the Spanish Inquisition couldn't dream up. Later, Sebastian comes to realize that The Boss is quickies in a bathroom stall because he gets off on almost getting caught, gets off on pain and expects the same of Seb, and the weirdest thing about it all is that nothing in Seb resists the utter surrender of his whole being to Jim, nothing shouts "THERE'S SOMETHING VERY WRONG HERE," because it's not surrender, ever: it's meeting the only other person in the world who hates it all as much as you do.