In Guns, Germs, and Steel, why does Diamond say that organized religions often "justified" kleptocracies?
The answer to this can be found in Chapter 14. Specifically, it can be found on pages 277 and 278 of the paperback edition.
In Chapter 14, Diamond says that the governments of states are all, to some degree, kleptocracies. What that means is that the governments take resources from the people they govern and use them for their own purposes. In that sense, we can say that the governments are stealing from the people (kleptocracy means something like “rule by thieves”).
The question then is why people stand for this. Why do they allow the government to take their money or some of the things they produce as taxes? One reason for this, Diamond says, is because religions can tell them that they should do so.
Diamond says that early states always have an organized, institutionalized religion. This religion tells the people that their rulers are gods or that their rulers have special connections to the gods. This makes the people feel that they have a religious duty to obey the rulers. That way, the religion makes it so that the government is justified in taking things from the people. The government is justified because its actions, in some way, represent the will of the gods.