Being connected to Sebastian is like being repeatedly stabbed by a mental and emotional knife. He knows everything that passes through Jace's head, and is quick to express his disapproval. For one thing, he doesn't like how he's still referred to as Sebastian, but Jace isn't willing to budge on that one; he refuses to share a name with this monster, even if he now has to share everything else.
The only thing Jace can compare it to is the paratabai bond, but even that doesn't cross quite as many borders as this does, and besides, that was a willing bond, and Alec, sweet, lovely Alec is nothing like Sebastian.
Degenerate, whispers the now constant voice in his mind, dirty, wrong, betrayer, he shall be the first to taste our wrath.
Jace winces, and pushes the voice away as best he can. He won't let it become a part of him, and he refuses to think ill of his family.
I'm your family, persists the voices, they are nothing but traitors.
It slices into his mind like a hit from Isabelle's whip (slut) but again he pushes it away. He's getting used to Sebastian's (no) mental tirade, and he can only hope that means he's getting better at resisting it, and not that he's weakening. He will not hurt the Lightwoods.
You could go back to them, murmurs the voice again, this time like silk and velvet, like a bed that instantly makes you want to lie down and sleep on it, tempting rather than the harsh berating he's become used to. And because of that it's so much worse.
His mind is suddenly flooded with images, unbidden but this time of his own doing, of Robert frowning down an him, but with a twinkle of a smile, and advising him against wearing slippery shoes when running about the Institute, of Maryse coming into the kitchen with a bowling pot and an apron over her usual skirt suit, of Izzy smirking at something he'd said, of Alec looking at him, crystal blue eyes open and earnest –
And then of himself holding Alec up by his neck, chocking the life out of him, and it is only the feeling of grim satisfaction that comes with the memory, which contrasts with his own horror that warns him it is Sebastian's.
The memory moves on, developing into the imaginary, where the hand succeeds and Alec is tossed, limp and cold, to the ground. And once again Jace resolves that he can't go back until this is resolved.
There is no resolution but our complete merging, snarls the voice once more.
Now that just sounds dirty, retorts Jace this time. He will not be a passive player in this game; he will not be used again. After all, Valentine taught them both the same lessons, so Sebastian should know he's not going down without a fight, even if it is to the death.
And then the image he's been pushing down all this time, swims to the surface of his mind's eye, of Clary, looking down, tear stained, at his dead body. He can't believe after all of that, after the Angel himself bringing Jace back they would only get a few weeks, he loves her –
And again, the wave of hatred hits, so very contrasting Jace's own emotions. It hurts enough to weaken him, but he can't help feel slightly triumphant.
Because if he can hear every though and feel every emotion of Sebastian's, that meant Sebastian was being subjected to his. This was still a battle from both sides.