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It's likely it was given to her by mistake initially.

That kiss was exclusive to Spawn and Karlach, AA couldn't have it because AA could only keep his relationship with Karlach if he skipped the transformation scene. This, by the way, proves that AA with its dominant notes didn't come out of nowhere.

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He says in dialogue that relationships are a form of control


He has no control over Tav, even when Tav sleeps with others. In real life though, most people have control over this in their partners.

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and a devnote for the attempted turning scene with Karlach even directly says he's thinking about controlling her.

Context:

Be careful, Astarion. My engine is running so hot. I won't be able to control it.
Hush love, it's all right. You don't have to worry.
Forget about your engine. Forget about control. I'll take care of everything.
trying to reassure Karlach, while thinking of how he's going to have control of her soon