What showed him that the war should never be won was what Germans were doing apart from fighting. He only hinted at those atrocities, but we had been taught about that at school and as children we were encouraged to read widely and to ask questions in our family. So we could guess what he might have been referring to when he could not go on.
He was the only one I heard about/knew who deserted. But I know that he cannot have been the only one.
Your words about duty remind me of Antigone before Creon arguing that his decree - to dehumanize one dead brother - goes against the laws of the Gods, against humanity.
I am glad that he left his troops and walked back. I admire him for it. His courage. His willpower. His stamina. His reasons.
I admire even more all those wherever in Europe who did their part in resisting an atrocious and inhuman regime. Who hid people, saved their lives, helped them. Knowing full well what they were risking. Doing what was right. Just like your characters in your story.