Normally I'd put this in tags but to avoid spoilers / also a long winded tag ramble... I don't know, something about this thread of 'right thing' and 'sacrifice' being both opposed and brought together through dark magic is really interesting to me.
First things first is the basis, I think, of what Callum believes "the right thing" in 7x06. In the scene, he's rejecting doing dark magic of any kind (the coin plan or otherwise) and saying that there has to be another way. He comes up with the Akiyu + archmage dragon plan, and believes this is the Right Thing in opposition to using dark magic. An alternative to dark magic, to doing the 'wrong' thing is his way out. Being a good person who does courageous things; doing what's right.
Then he realizes after Akiyu is dead that dark magic is the "another way" through. It's the only alternative left. So he asks Runaan to do what Rayla preached in 6x03: to sacrifice one life for the greater good. He's not sacrificing her (rather their life together, but she'll still get to have a life), but himself. He's given up doing the 'right thing' for himself, accepting that he has to do the 'wrong' thing. His way out is still good people doing courageous things, doing the right thing, but no longer includes himself. Instead, the right thing is for them to take him out of the equation. After he corrupts / ruins / dooms himself.
What you did was hard, but it was the right thing. You know that, he tells Rayla. I know you do.