I decided to turn this question over to Sarah Rees Brennan, as she is my on hand Sebastian expert and an objective eye.

“Hahahaha objective. What Cassie means is, we are in a French manor drinking grapefruit juice and I am freaky about demon babies, so she thought it would be hilarious to turn this over to me. (I like them. Poor precious babies. They didn’t ask to be demonic, did they? Izzy wizzy satanwookums.)

Well, okay. Let’s first examine Valentine’s plan. Actually, I don’t think it was as good as Sebastian’s.

Valentine’s plan was to dial 1800-ANGEL and be like ‘I just met you/and this is crazy/but I’m a magic racist/destroy all this icky magic people maybe?’

And since he was a loonball who’d just decided based on little evidence that angels would totally be into his whole agenda, Raziel went ‘So I created Shadowhunters to deal with demons because I didn’t want to be bothered with all this? And now you’re bothering me to smite people. OKAY, I’LL SMITE SOMEONE. IT’S CALLED DELEGATION BUDDY, LOOK IT UP!’

Raziel was extremely upset to see both Valentine and more recently, Simon. If the Little Demon Who Could summoned him, one can only imagine it would go poorly.

Sebastian has no intention of getting angels involved. Instead, his plan is to get demons involved (so far, Lilith, very helpful, clearly would’ve run to the store to buy Sebastian his favourite juice. I believe Sebastian’s favourite juice is Blood of the Innocent). And to create an evil army to reorder the world in his image (so far, also pretty successful, people have been eviled up a treat!)

Sebastian doesn’t actually want to burn down the world. That’s just his snappy propaganda line. The world has all his stylin’ clothes in it. Sebastian wants to *change* the world, to honour his father’s memory (oh poor Sebastian, your daddy never loved you like he should’ve, but he was the only one who seemed to love Sebastian at all), to suit himself better (world full of skilled demon hunters = obviously not ideal for Mr Tangy Demon Flavour) and to spare himself from what is clearly a pretty all-encompassing loneliness.

His plan is a fairly standard one, in two ways. a) he is a monster, and he wants to make more monsters like him so he won’t be alone forever. Classic. Just like Frankenstein’s monster.

b) He is an evil mastermind, and he wants to raise an evil army to lay waste to his enemies so he can run the world as he sees fit. Classic. Just like Skeletor and Emperor Palpatine. [Edited by Cassie to add: Also like Gargamel from the Smurfs.]

His is a two-pronged plan! But both prongs are fairly easy to get. What are we doing tonight, Sebastian? The same thing we do every night, Sarah… collect demony friends for demony playdates and try to take over the world with an evil army.

And of course, what else can Sebastian do? The world HAS to change, for him, because his father made him something demony and strange, and because of this—which is in no way his fault—his mom is repelled by him, and his sister and brother (uh, justifiably, obviously, Sebastian has done them wrong) don’t want anything to do with him.

Of course he wants to shake everything up. As Satan says in Paradise Lost, ‘Evil, be thou my good’… because good so far has been no good to Sebastian: both nature (in the form of Valentine’s meddling ways) and nurture (ditto, Valentine!) have put him in this Catch-22 of Evil.

If Jocelyn had managed to take Sebastian with her, and raised him in Brooklyn with Clary and she and Clary had both tried to love him and teach him morals, I’m not saying Sebastian would have been a sweet demony Boy Scout, but things might have been different than they are now… But I must stop there, as if Cassie finds me actually writing a fanfiction entitled City of Precious Little Demon Baby in her tumblr askbox, I fear she will beat me about the head with a baguette.

In conclusion, I hope you now have a better understanding of what Sebastian’s plan is and what motivates him to accomplish it. ;) And also, shed a tear for a demon baby who never had a chance…

I LOVE DEMON BABIES OKAY? IT’S A SICKNESS.

” [Edited by Cassie to add, Word, it so is. Well analyzed, though.]