Pizarro vs Atahuallpa

Maybe Guns Germs and Steel fans can help me with this. In Chapter 3 Diamond asks the questions "How did Pizarro come to be at Cajamarca? Why didn't Atahuallpa instead try to conquer Spain?". Diamonds answer, paraphrasing roughly, is that Spain had guns, ships, literacy, and centralized political organization. The Incas only had the latter, and this actually ended up working to their disadvantage.

It seems to me that Diamond confuses 'how' and 'why' questions. When we ask 'how' we might answer with technical factors that explain. 'Why' is more of an ethical issue. Just because Spain had guns does not explain why Pizarro vanquished Atahuallpa. Possession of technology [can suggest/influence but] does not determine its use. When we make sense of, say, Nazism, we cannot merely point to military technology and centralized decision making, even though these were an important part of Nazism's history. Questions Diamond avoids are "Why did Pizarro come to be at Cajamarca? Why did an imposition of Christianity mean a slaughter of the Incas? How is it that imperialism came to be seen as serving the church?"

I'll keep reading Guns, but I remain a skeptical reader...