There's also a part of Harry's conversation with Ebenezar in the last chapter of Changes.
"Hell of a hard thing to do."
"It wasn't hard," I said quietly. "Just cold."
"Oh, Hoss," he said. There was more compassion in the words than you'd think would fit there.
Harry has been paralyzed from the waist down, and is succumbing to despair, and prays to the archangel Uriel to help him. Uriel arrives and says he can't, but then reminds him that he does have a couple of other avenues open to him to save his daughter. Finally, he makes the choice he's been avoiding for years.
"For you, little girl. Dad's coming. Mab! Mab, Queen of Air and Darkness, Queen of the Winter Court! Mab, I bid you come forth!"
When Harry that he has to kill Susan to win the day at the end of Changes, Butcher makes clear how horrible it is for him.
I used the knife
I saved a child.
I won a war.
God forgive me.
What makes it extra powerful is that for the majority James Marsters' reading of the audiobook, he's fairly calm. Sure, there are scenes where he tinges his reading with the anger, disbelief, weariness, pain, and fear that Harry has to be feeling, but when he gets to "God forgive me," the line is delivered with a heartbreaking half-sob. It almost sounds like Marsters himself was breaking down crying while reading that chapter.
What also makes it even more poignant is that Harry has always kept his distance from religion of any sort and has expressed his rage against God on numerous occasions. So to see him breaking down and begging God for forgiveness is very painful.
Hell, Marsters' reading the entire last chapter. Especially when Harry asks Murphy to take Maggie to Father Forthill and put her someplace safe, and that he doesn't need to know.
A small but powerful moment afterward: the Leanansidhe says that she will bury Susan with all the respect and honor that Harry would wish to do himself, and even gives her word that she will do so without expecting anything from Harry in return - something incredibly rare among The Fair Folk. But the hammer comes from Harry's thoughts immediately afterward.
''I looked down at the child, a sleepy, warm little presence who had simply accepted what meager shelter and comfort I had been able to offer. And I thought my heart would break. Break more. Because I knew that I couldn't be what she needed. That I could never giver what she had to have to stand a chance of growing up strong and sane and happy. Because I had made a deal. If I hadn't done it, she'd be dead—but because I had, I couldn't be what she deserved to have.
And Maggie's scared of her own father's face. Oh sweet merciful God.