Just some comments from what the OP wrote:
I don’t think you can be forced to sell your soul to a fiend. It’s a conscious choice. That means Karlach may just be having “buyers remorse” which doesn’t make her all that much better than the fake followers of Tyr.
That is the general rule, but one has to make room for narrative exceptions. People being sacrificed, parents offering up their firstborn (or entire lineage), people getting tricked or blackmailed, and so on.
