Doc can't believe that he's actually doing this. After so long of trying and failing, after so much death - at his hands, as well as other's - it is finally working.
Maybe he hasn't found a solution to the problem, but they've got someone who has and that someone is willing to teach him.
But only for a price.
And that makes his heart sink, because honestly he thinks the world of Wanda. She doesn't act like a Soul and most of the time it's actually hard to remember she is one. She tells them all stories, of Souls, of the lives she's led, and she does it all like she wants to be there, even back when she was more of a prisoner in those caves than they are.
He doesn't want to do this.
But he has to.
And he hates himself for it.