I agree pretty ,much with Caerus. I woulndn't say that Jack IS evil though; just that his inner Beast takes over him and makes him savage. Under his leadership, the other boys' Beasts take over, too. Even Ralph's beast comes out when he takes part in a hunt and stabs the boar.
Yes, Ralph is a better man, whose rule is all about rescue, i.e. what is best for the boys. In contrast, Jack's rule is all about his power. The irony is that Jack's rule is more effective with the boys, because he punishes disobedience while Ralph does not.
Oh--the notion of Beast being only in little boys is incorrect. That Beast is in all of us. At the end of the novel, when the boys are taken off the island, ARE they really rescued? They are removed from their little war to a ship that is fighting the same war on a much larger scale. Hence, the island is a true microcosm of our planet.