There's a part of him that wants to say that it doesn't have to end like this, that there are other ways of solving this problem (like it's a fucking breakup-which, well, isn't as inaccurate a description of the situation between the two consulting geniuses as one might think), but he doesn't, is smart enough to know he shouldn't, but it still hides, unspoken in every gesture and glance, and he knows Jim hears it anyway. Sebastian knows that it does have to end like this, and that it wouldn't matter if he did say something, because he isn't brilliant but he can figure things out, knows these moods, remembers how stories always go.
And he knows how broken glass sharp Jim's mind is, but, the thing about broken glass...
So it does have to end like this, and always has, and he knows it, always has, didn't need it spelled out for him.
It's all a story (well, Jim says so, and Sebastian thinks he's crazy, but he nods and agrees, because he is crazy) and they're the villains from most perspectives, and there's no happy ending here-but wasn't that rather the point. And Sebastian doesn't say anything, just watches Jim take a gun from a drawer, and it's not his place to say anything and what could he say anyway.
Buys a children's book, molasses, more sealing wax- "not your fucking servant," he'd said, low but more audible than he'd intended, and the fingers tight on his throat, nails pressed hard (and sharp, that was always surprising) against pulse points corrected that quickly-reminded him that he was more than a just hired gun now, had been for a long while, was whatever Moriarty wanted him to be- and he knows that, he really does, but, well, there was a reason that none of his officers were sad to see him go (well, more than one reason).
Threatens some people, kidnaps others (if you ever want to see your children or your wife or your mistress or your dog or blah blah blah it got so boring after a while and he'd reminisce about the days when this could all be done with a knife against the throat, a whispered threat, a boot to the ribs), makes sure everything goes smoothly and everyone does what they're told, and if he were more poetic he might compare himself to the author of this story, or the editor, making sure everything ran the way it should, through to the end-but there was a reason his books never really sold, and besides the story was written a long time before then (in a small, crowded house in Dublin, not that Sebastian knew much about that).
And he waits in the stairwell, too, gun pointed at the pet of a doctor the junkie genius has taken up with, and he listens, and he watches. Tenses a little when he finally does hear the shot- like seeing how many pages are left in a story and realizing how much shorter than your expectations the story must actually be- but by then he's calculating. Wondering how long this can be dragged out, waits for a look either of relief or anguish to cross the poor doctor's face.
Finally there's sadness there, and then a mad, useless rush to a body that can be nothing other than dead, and he calmly and carefully packs up his gun. This one is named Vera, and he wouldn't tell anyone that on pain of death. Jim had said that endings required an extra flourish, and to use his favorite gun- well, second favorite. The mad bastard had lifted the first favorite out of a desk drawer the afternoon after his trial, extra fucking flourish right there.
"It's always in the details, Sebby," Jim had said one evening, sprawled out on the couch in a splay of limbs, looking upside-down at Sebastian cleaning his guns at the kitchen table. "Without them every story's the same, good and evil and personal growth and all that shite." Sebastian had winced slightly at his most hated nickname and grunted in acknowledgment, not looking up. From Sebastian's perspective, it had played out the same way most of these things did, for all Jim's effort with "details." He'd be disappointed if he knew it had happened that way not with a bang but a whimper-literarily speaking, of course.
He walks up the rest of the staircase to the roof and leans for a moment against the door, lighting a cigarette. Others had already come up, starting to take care of the mess and clear everything out- Richard Brook had gone abroad, hoping for a big break, or something equally rubbish; Brook wasn't part of the story, those details didn't matter. He walks over to the corpse of his employer, and looks at him as if searching for something, but his facial expressions have never been clear in the first place. After a moment he mutters, "idiot," kicks at his ribs half-heartedly, and flicks ash on Jim's face, a few specks landing on his glassy, still-open eyes.
The villain always causes his own destruction, in one way or another, doesn't he, and a broken glass mind will cut at itself until it's destroyed. Because the thing about broken glass is that it cuts everyone who holds it, sooner or later.