I like the archetype of a character who once did cruel things but has gone through so much they have gained enough wisdom to escape being what they once were, probably like Chrno. Always a fascinating character.
The astral line description being that it is a stream of life that sprinkles down creating souls on earth, and when life ends it joins back into the stream is something that I don't think the anime described nearly as well as this.
I also wonder if hearing Rosette say she wanted to be a doctor for his sake was the push to make him agree to go with Remington.