These boys could have been friends—they look at each other with "shy liking"—if they weren't bitter enemies. Compared to Jack's impulsiveness and violence, we see Ralph's fairness, concern with social order, and concern for the future more clearly; and we also remember that good and evil are part of everyone.
Just when you thought Jack was bad enough, Roger comes along and one-ups him. We dislike the things Jack does, but we can see an innocent boy in there the whole time. Because of his connection to Ralph, we watch his descent into primitivity from a starting point we could all relate to. Roger, on the other hand, is a rotten apple from the start. He shows us that, while in many cases darkness is part of man’s heart, in some cases that darkness is in control most of the time.