Thanks! No, I can answer, because it isn’t anything not covered in CoLS.  ”It stands to reason that this is because of the demon blood they’ve ingested” — well, sort of. It’s because of the demon blood they’ve ingested from the Mortal Cup. That’s why Sebastian spent so much time and effort creating a double of the Mortal Cup. Simply having them drink demon blood doesn’t create dark Shadowhunters. Only drinking from the Mortal Cup creates Shadowhunters. Only drinking from the dark Cup creates dark Shadowhunters. Humans who ingest angel blood don’t become Shadowhunters unless they drink it out of the Mortal Cup — that’s why the Mortal Cup is important. Sebastian wanted an army of dark Shadowhunters, which is why he spent a large portion of the book creating a dark Cup. Otherwise he wouldn’t have bothered, and would just have fed them all Lilith’s blood out of a Coke can, because Sebastian never expends effort to no purpose. He is basically lazy.

Dark Shadowhunters aren’t people who have drunk demon blood. They are Shadowhunters who have gone through a ritual involving the Mortal Cup that has transformed them into what the Shadowhunters would have been had Jonathan Shadowhunter initially summoned a demon, not an angel: i.e. a completely different race of magical warriors.

*scratches head* I don’t think it’s surprising that anything about Sebastian is odd. I think it’d be more surprising if he ever did anything normal.

These kind of questions are always hard to to answer because they skate into what Libba Bray calls “Why is your cat your cat and not some other cat?” territory — that well, sometimes characters just are the way they are because if they were otherwise, they would be a different character.

That Sebastian has a strange fixation with Clary is one of the first things we ever find out about him. He has it from the moment we meet him in City of Glass. He follows her around, and kisses her, and whinges about her to his (their) dad:

“Clary wasn’t at all like I thought she’d be,” Sebastian went on petulantly. “She wasn’t anything like me.”

“There is no one else in the world like you, Jonathan. And as for Clary, she has always been exactly like her mother.”

“She won’t admit what she really wants,” Sebastian said. “Not yet. But she’ll come around.”

—City of Glass

From this, we know that 1) Sebastian is fixated on the idea that Clary is like him — maybe his only chance at finding someone in the world like him, which is his idee fixe — and 2) he hasn’t given up hope that she will turn out to be like him/a compatriot/on his side.

So why would that change? That Sebastian is fixated on his sister in a weird way has been part of his character since Day One, since part of his character is that he is unique — “there is no one else in the world like you” — and that he both likes that and suffers loneliness because of it. And that Clary is his main hope for “someone like him” has also been part of his character since he first showed up.

I think it’s also interesting to be asked why I “made Jonathan/Sebastian love Clary” when it’s entirely a matter of interpretation that he does love her at all. He never says he does. Neither does anyone else say he does. Why assume he does when it’s an open question whether he can love at all? (I’m not really asking you, asker, just tossing the questions into the air.)

But indeed, of all the people in the world, Clary and Jace are the only ones who even seem to register to Sebastian as actually existing and possibly mattering. And in both cases he’s obsessed with either finding or creating “someone like me” — he does it with Jace with literal possession, and with Clary with mind games — “You have a dark heart in you, Valentine’s daughter” — and physical attempts to control and possess her. But that’s what he wants, basically, someone like him, and he thinks he can have that only with Jace or Clary.

I mean, I am not saying, I suppose, one way or the other —— given that the series isn’t over, we really don’t have any idea what Sebastian/Jonathan’s final plans are, or what he thinks he feels — whether he thinks he loves his sister or whether he thinks he doesn’t. He talks and thinks a lot about belonging and owning and controlling but not ever about love. He’s obviously obsessed. He wants to hurt and upset and control her. And he seems willing to kill her if required. But it does seem like he’d rather not? …I leave it up to you whether this constitutes love or not. 

So, I saw the question about Sebastian’s capacity of love and decided to dig deeper. I always portrayed him like the damned hero, who finds salvation or, if is to late for that, at least some kind of forgiveness through the love of someone. Any chance on this or is Sebastian doomed to remain and perish as the soulless antagonist? 

But is he soulless? *points up* I mean, either he can love or he’s soulless, can’t be both, no? And you can be evil, but not soulless — look at Mortmain and Valentine. Jonathan has a soul. If he didn’t, he couldn’t bear Marks. People with souls do horrible things all the time. He may have a capacity for love and human feelings. He may or may not decide to ever use it. He may or may not already have done such terrible things that salvation is not possible. 

I think lonely monsters fascinate us all because we all know what it’s like to be lonely, and so, however horrible they are, there may be a creeping part of us that feels a twinge of sympathy. As Clary says:

She had seen Sebastian looking at Jace, even at herself, and knew there was some part of him as echoingly lonely as the blackest void of space. Loneliness drove him as much as a desire for power—loneliness and a need to be loved without any corresponding understanding that love was something you earned.

Anyway, is there any hope for Sebastian? Well, we have another whole book to find out.

Does it seem like he does?

(That seems an advanced level of love for someone who needs some kind of Murderers Anonymous 12-step program for Not Killing People, but I suppose it’s open to interpretation. I guess being able to love doesn’t actually make you good.)