The answer to this question can be found in Chapter 14. A good place to start is on p. 276 in the paperback edition of the book.
Diamond says that some governments are kleptocracies because they take from the poor and give to those who are rich and powerful. The word “kleptocracy” means something like “a government run by thieves.” Diamond is saying that some governments use their power to steal from the poor. As he says,
At worse, they (some governments) function unabashedly as kleptocracies, transferring net wealth from commoners to upper classes.
The point that Diamond is trying to make in this chapter is that large and complex societies always end up as kleptocracies to some degree. He says that civilizations with lots of unrelated people living together can only function if they have strong centralized governments. All of these governments will inevitably take money in the form of taxes and use if for their own purposes. Some, like the government of George Washington, will be seen as relatively benign because they spend some of the money they get on the common people. Others, like that of Mobutu in Zaire, are pure kleptocracies because they do not spend the money on anyone but themselves.
This is important to Diamond’s overall argument because it is the centralized, kleptocratic governments that can create large armies and use them to conquer other people. This is another advantage that large societies based on agriculture have over hunter-gatherers.