The citizens of Elturel swore a sort of oath-of-alleigence that required them to defend the city. This oath was corrupted so that the citizens had unknowingly sworn themselves into Zariels service.
So it seems that it is possible for Karlach or Anders to have been bound to Zariel without their consent.
Interesting idea.
Anders I am sure sold his soul outright. He even says so when he’s caught in his lie.
It’s possible Karlach was caught up in the oath but her story suggests she was fighting the blood war for quite some time. And it doesn’t explain why the other tiefling refugees aren’t targeted personally by Zariel.
It does sound like Karlach is much older than Anders, and she says that her soul was indentured in Baldur's Gate, not Elturel -- she sounded like a jaded veteran who had spent at least a couple of centuries climbing the ranks in Avernus, while the fall of Elturel and Anders' oath is relatively recent in-game.
This means two things:
1) Anders, as a human still alive, is not yet conscious of what it exactly means to be a Zariel soldier IN the Blood War. He's still in the honeymoon stage, so to speak.
2) Karlach is one of the two (potential) companion characters that were around when Baldur's Gate 2 took place... the other one being JerkMagistrate!Astarion, pre-bite. Astarion, who has what looks like an Infernal seal carved on his back. I'm betting my two soulcoins that their Act 2 and 3 questlines are linked.
