This is a great question and it is important that you say that both themes of human nature and politics/government are important, because both themes are important in the book. In light of this point, I do not know why you must choose only one. It is a false binary to have to do so. An essay that explores both would be best. If this is not an option, I would attempt to write an essay on human nature for two reasons.
First, all the characters of the book are boys below the age of thirteen. This point can be significant, because you can argue that they have not been socialized as much as adults. Therefore, what comes out is their nature. In short, you can argue that it is human nature in pristine form.
Second, the age of thirteen (and many boys are even younger) is rather young to form a rational political or governmental structure. The two groups that are formed in the book can hardly pass for government. So, the point that is left is once again an emphasis on human nature.