I mean, I see your idea, I just think that Gortash wouldn't want to deal with a ticking bomb (in all the meanings of the word - she's set to kill him AND her engine is on its last fumes) like that without a really good reason. And since I don't think Zariel would pay again for what she has already bought from him once, there's really not much for him to gain except problems.

Also with Astarion and Shadowheart giving them over saves you a fight, and Gortash mostly wants to work together from the get-go.

Now that I think about it, out of characters we do have in the game ('cause I see your point, Zariel would have to be added) I think a potential betrayal option for Karlach could come from Mizora, actually. She has something to gain (Zariel's favor for returning her favorite toy to her) and she has something you want from her (Wyll's pact or the Duke's life).

(this whole conversation is such a prime example of the fact that we want not just a happy ending but a full quest with options I swear)

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