For an essay like this, as long as you are comparing the beginning and the end, you might as well contrast it as well. It will give you more to write about, and it allows you to set up a two-sided thesis statement, something like: Although the boys on the island start out with the best of intentions toward each other, the lack of formal leadership, training, and mentoring causes the boys' society to devolve into total chaos with no respect for anything except strength and violence.
This statement allows you to discuss how upon crashing, the boys try to set up a sort of governing democracy ruled by reason. You can talk about what both Simon and Piggy bring to the table in terms of a stabilizing effect. You can also discuss and provide evidence as to how Ralph is a good leader.
Then the second half of the thesis statement allows you to walk your reader through the step-by-step break down of the boys' society and government. You can defend what you believe is the cause for this. Maybe you think it is lack of adult supervision. Maybe you think it is the presence of a few "bad apples." Maybe you believe that all people are inherently evil. That last one might be easy to support since Simon had his conversation with the "Lord of the Flies" and that's what he was told. In any case, the novel does not end on a happy note. Yes, rescue does show up, but everything leading up to that is vile and murderous.