It's not that Clary likes him, she reasons to herself, trying to find sense in madness. It's just that she likes the way he mewls her name when she's over him, touching him in all the ways he wants to be touched, the two fucking roughly in closets or the beds of the recently murdered, still coated in their blood. It warms her up inside, riding him to completion as she feels the blood dry against her skin.
It didn't start like this, she recalls, distantly. He's moaning, pressed against a wall with more blood splatters than paint, and his pretty open mouth makes it hard for Clary to remember things properly. It started…
It started with her and Jace, but Jace was being controlled by Sebastian. Kissing Jace was just kissing Sebastian, so why not cut the middle man out? Just because she wasn't supposed to kiss her own brother? What was the point, anyway, when kissing her boyfriend and kissing her brother felt the same? Blood called to blood, and Clary didn't want to deny herself what she wanted - and she wanted Sebastian.
So Clary broke things off with Jace - and since Jace didn't have any more holding power over her, Sebastian got rid of him. His blood was still dry on the floor of the apartment, and his corpse… Well, they had thrown it in a river, if she recalled it well. Several rivers. Maybe.
Clary knew, in some deeper level, that this was wrong, but killing Jace - watching as life left his eyes, once so trusting and full of love for her now filled to the brim with heartbreak - had been more fun than expected.
Switching sides had never been so easy. Clary joined her brother's murderous sprees, bathed in blood, and fucked him after it. He liked it, and she even more so. What else can there be to it?
Her father - thinking of him as she makes Sebastian cum feels wrong, but yet here he is, almost, almost, yes, Clarissa, - had said she had more angel blood than any other shadowhunter alive. Clary feels like he was partially wrong; no one who's so good and gracious and holy goes down to the evil side so easily. Maybe her blood was that of a fallen angel. This must be the reason, Clary reasons to herself. She fell because she was fallen from the start, not because she's in love with her own brother or anything like that.
He goes limp in her arms, spent in her hands, and Sebastian looks at her with adoration in his pitch black eyes. Hers must look a similar colour, in this dark room that smells like copper.
She indicates with her head the outside, where she can hear nothing but silence, and he grins, kissing her more softly than one would expect, before going outside the room they are in. It's almost like they're two teenagers making out in secret.
Wait, they are, it just has more blood and guts. Clary grins to herself and cleans her hand with her mouth, licking his come out of her fingers, before following Sebastian out, humming as she steps in puddles of blood.