Quotes and dialogue mainly taken from City of Glass itself, credit goes to Cassandra Clare, not me.
Sebastian hated people who eavesdropped on others' conversations. Especially when the conversation had nothing to do with them. From the very start, he had already sensed the angel boy's presence, thanks to the heightened senses gifted by the demon blood that flowed in him. It took him every single ounce of willpower to concentrate on what his father was saying to him, instead of exposing Jace right from the outset. He would deal with Jace himself, make his father proud for doting on the right child- the more powerful and ruthless child.
As soon as Valentine disappeared into the darkness, Sebastian called out to Jace, "You might as well come out, Jace. I know you're here." He sensed a sudden stop in Jace's movement, indicating to him that he caught Angel Boy by surprise. Jace came to his senses rather quickly, much to his chagrin. As he headed towards the tunnel entrance, Sebastian stepped right in front of him, already reading his plan from his actions.
"Really, you didn't think you were faster than me, did you?"
Angel boy shot back some insignificant remark, but his heart was beating so unevenly that Sebastian could hear the fear in his heartbeat.
He smiled, "I could hear your heart beating . When you were watching me with Valentine. Did it bother you?"
"That you seem to be dating my dad?" Jace shrugged. How dare he have the nerve to dismiss what I said? "You're a little too young for him, to be honest."
"What?" That comment struck a raw nerve. How dare he doubt Valentine's affections for me? How dare he doubt that Valentine loved me more than he loved Angel Boy? He allowed himself a moment of anger, before composing himself again. Slowly, he taunted Jace about his upbringing, playing on Jace's issues with Valentine's abandonment.
Realization dawned on Jace's face as he understood where Sebastian was getting at. "You, Valentine's your father. You're my brother."
By that time, Sebastian was already behind him, his arms around Jace's shoulders. Too slow, he thought, way too slow. "Hail and farewell, my brother." He spat and tightened his arms, cutting off Jace's air supply.
When Jace fell unconscious, Sebastian took his time to tie Jace's hands behind his back. He watched the sky as it turned dark. Midnight, everything would change at midnight. His father's army was to rise soon and then they would rule the Shadowhunter world hand in hand. No one would doubt his father's powers. Not his either.
The sound of Jace's coughing broke him from his reverie. "Awake, little brother? He knelt in front of Jace, playing with the knife in one hand, and the length of rope in the other. "Good, I was afraid for a moment that I'd killed you a bit too early."
Jace turned his head to the side and spat out a mouthful of blood. "Waiting for a special occasion to kill me? Christmas is coming."
He laughed internally. Poor little angel boy. His only skill is a smart mouth, and that wasn't even from our dear father. Pathetic, plain pathetic. "You have a smart mouth. You didn't learn that from Valentine. What did you learn from him? It doesn't seem to me that he taught you much about fighting either."
Sebastian got to his feet. Finally, he was going to summon the demons, as part of their plan. But only earlier. He wanted Jace to see this, to show him how powerless he was to stop anything Valentine had planned. He drew the sword from his sheath, said a few words in demon language and the gates opened. "Watch and behold Valentine's army", he yelled.
A cloud of darkness surged from the stalagmite overhead, demons rushing out of the portal to do Valentine's bidding. They rose shrieking, howling and snarling, a boiling mess of claws, talons, teeth, burning eyes and the stench of rotting flesh filled the chamber. Sebastian watched triumphantly as the demons pushed past one another, leaving the cave pitch black for a while as they made their exit.
Sebastian made it a point to tell Jace Valentine's plan, to watch the despair on his face, the disbelief and to watch hope drain out of that little angel boy's face. He hated those golden eyes of his. It was a reminder of how weak the little angel boy was, with the blood of an angel flowing in him. Yes, he knew. He knew how different both of them were. And he was proud of that difference. Because he was the stronger one, the more superior one. The chosen one to lead Valentine in his quest. The one with demon blood.
"Little angel boy, you're a fool, aren't you- just like my father always said."
"Why do you keep calling me that? Why are you blathering about angels-"
"God, you don't know anything, do you? Did my father ever say a word to you that wasn't a lie?"
"How do you know he wasn't lying to you?" Sebastian wanted to wipe that annoying smirk of Jace's face.
"Because I am his blood. I am just like him. When he's gone, I'll rule the Clave after him."
"I wouldn't brag about being just like him if I were you."
"There's that too. I don't pretend to be anything other than I am. I don't behave as if I'm horrified that my father does what he needs to do to save his people, even if they don't want- or if you ask me, deserve- saving . Who would you rather have for a son, a boy who's proud you're his father or one who cowers from you in fear and shame?"
"I'm not afraid of Valentine."
"You shouldn't be. You should be afraid of me." He lowered the sword he was holding in his hand to the spot right above Jace's collarbone, and with minimal effort and maximum restraint, he nicked Jace's Adam's apple.
"So now what? You're going to kill me while I'm tied up? Does the thought of you fighting me scare you that much?"
Instantly, a sense of revulsion and disgust flooded Sebastian. He was merely playing with his prey. It made the final blow seem so much more intriguing. How his prey would beg for his life, or better still, not see what was coming and then he would see the light in his prey's eyes drain out slowly but surely. Just like that ignorant little Lightwood boy. The sight of blood was gloriously beautiful. Fresh red blood flowing out of that boy's misshapen skull, it was a feast for his senses.
"You are not a threat to me. You're a pest. An annoyance."
"Then why won't you untie my hands?" Because I am not through playing with you, little angel boy. I want to watch you suffer, watch you lose hope, watch your life bleed slowly out of you.
"I'm not a fool, and you can't bait me. I left you alive only long enough so that you could see the demons. When you die now and return to your angel ancestor, you can tell them there's no place for them in this world anymore. They've failed the Clave, and the Clave no longer needs them. We have Valentine now."
"You're killing me to deliver a message to God for you? You're crazier than I thought." Jace shook his head and Sebastian felt the sword scraping the skin of Jace's throat, drawing more blood. He smiled at Jace's stupidity and pushed the blade in slightly deeper, until he felt the tip graze Jace's windpipe. That ought to shut little angel boy up.
But Jace didn't shut up. He continued taunting Sebastian about how Valentine would know what he did, how Valentine would find out that he was too cowardly to fight the other son on equal footing. He stared down at the little angel boy, hatred boiling behind his eyes with each passing moment, until something snapped in him. With all the hatred and a fraction of his inhuman strength, he drew the sword up. In a whirling arc, the sword came down swiftly and he felt the sword connect with flesh, muscle and then bone- a melody so beautiful that he couldn't help but smile reverently at his masterpiece.
Sebastian hated the waiting. He hated how Jace would pass out on him from a minor wound. "Get up! You have five seconds before I kill you from where you are."
"Why did you bring me out here?"
"Two reasons. One, I enjoyed knocking you out. Two, it would be bad for either of us to get blood on the floor of the cavern. Trust me. And I intend to spill plenty of your blood."
Sebastian watched as Jace felt for his weapons, weapons which he had already tossed to one side after dragging Jace's limp body out of the cavern. He smiled at the look of horror on Jace's face.
"Not much of a weapon, that." He grinned.
"I can't fight with this."
"What a shame." He walked closer to Jace, taunting with the sword in his hand, the tip of the hilt precariously balanced on his hands. It was bait, bait for the little angel boy to attack him. And of course, Jace was dumb enough to take the bait. He swung his arm and punched Sebastian hard in the face. Sebastian felt his bone crunch under Jace's knuckle and he was sent flying back from the force exerted. He hastily wiped blood from his nose, waiting for Jace to make his next move. "So go ahead. Kill me already."
Jace picked up the sword, raising it up to strike a blow. Something which Sebastian had already expected. Taking Jace by surprise, he leapt up into the air, disappearing temporarily with the speed of his movements, and kicked Jace's hand. The sword spun out of Jace's grip and landed in his hand. With deadly aim, Sebastian laughed, and slashed out with the blade directly at Jace's heart. As Jace leapt backward to avoid it, Sebastian was faster. The tip of the blade sliced open Jace's shirt, leaving a shallow cut down his chest.
Sebastian chuckled. The game was getting interesting, but only marginally. The little angel boy was no match for him, regardless of the angel blood flowing in him. He slashed the sword towards Jace, who leapt up and caught the nearest tree branch. He looked up at Jace, furious as Jace's dagger came falling towards his swordarm. The sword fell to the ground and he jumped up to the branch beside Jace, annoyed for letting Jace one strike against him.
"That was fun. But now it's over." He flung himself at Jace, knocking the both of them out of the trees and onto the incline of the river. As they fell through the air, they tore at each other, throwing punches, backhanding each other across the face until they landed hard on their backs. Jace grabbed Sebastian's throat once they landed, and in response, he grabbed Jace's wrist, twisting at a weird angle until he heard the bone snap.
Jace fell back into the river and at this moment, Sebastian caught a glimpse of Jace's dagger. He picked it up with his right hand and half-knelt on Jace's chest, grinning with utmost glee and satisfaction. He would finish this once and for all, no more games, no more playing, no more taunting. Just one strike at little angel boy's heart and he'll be gone for good. The point of the dagger rested squarely on Jace's chest.
"You've had your chance, Wayland. Any last words?"
The little angel boy seemed to react strongly to being called Wayland. It didn't matter to Sebastian. He pressed his weight onto the dagger, piercing Jace's skin with its tip. He hissed at Jace, telling him how naïve he was to think himself Valentine's son. He was merely a changeling, a baby Valentine cut out from a dying woman for his experiments. But little angel boy was too weak for anything, and so Valentine offloaded him to the Lightwoods. Valentine had never loved him at all. He was so certain of it.
"I am Valentine's son. Jonathan Christopher Morgenstern. You never had any claim to that name. You're a ghost, a pretender."
"You're the one with the demon blood. Not me." Jace choked from the pain in his chest. Sebastian smiled to himself again.
"That's right." The dagger slid another millimeter into Jace's flesh. "You're the angel boy. I had to hear all about you. You with your pretty angel face and your pretty manners and your delicate, delicate feelings. You couldn't even watch a bird die without crying. No wonder Valentine was ashamed of you." He sneered, the words tumbling out his mouth like acid.
"No." The little angel boy still had the cheek to argue with him. "You're the one he's ashamed of. You think he wouldn't take you with him to the lake because he needed you to stay here and open the gate at midnight? Like he didn't know you wouldn't be able to wait. He didn't take you because he's ashamed to stand in front of the Angel and show him what he's done. Show him the thing he made. Show him you. He knows there's nothing human in you. Maybe he loves you, but he hates you too-"
Lies, all lies. He doesn't know anything. How dare he question my father's love for me? How dare he smear the love my father has for me? How dare he, insolent little fool?
"Shut up!" Sebastian screamed as he pushed the dagger all the way down into Jace's chest, twisting the hilt. He felt the dagger slip through Jace's rib cage and the tip pushing down into his still beating heart. Not for long. That heart will stop beating very soon, and I'll be here to hear it slow down by the minute. To hear his life come to an end. Jace arched back with a bloodcurdling scream and Sebastian rejoiced, as blood started to pool in Jace's mouth. The smell of fresh blood didn't disgust him. Instead, it made him happier. The coppery tang of blood was like the scent of flowers to him, welcoming, refreshing and the bearer of something good.
He beamed down at the little angel boy. He was the superior one. Always was and always would be.