Luckily for my tired eyes, this is a relatively straight forward question to answer.
Think about the motivations and actions of the characters being listed as representing civilization:
- Piggy is a geeky intellectual who respects authority and the power of the rules to bring order to things. He is not violent and enjoys the "democracy" of the island and plays within the traditional "political game" the boys find themselves in.
- Simon is more sensitive, intuitive, and emotional, but he is not violent either. Though not as intellectual as Piggy, Simon feels a good deal of empathy for the other characters.
- Ralph is, for a while, sort of President of the Island. He values democracy, hard work, and collective action. He is interested in huts, latrines, and signal fires...all hallmarks of civilization. Above all, he is interesting in getting everyone off the island and back to the civilization they have left.
Now look at the characters being listed as savages:
- Jack paints his face in an evil mask, overthrows the democracy of the island, and reduces things to a creepy tribalism. His focus is not on building but on hunting. He is bloodthirsty and enjoys slaughtering the pigs. He is rude to Piggy and only tolerates Ralph because Ralph is tougher...in the end, though, he even turns on Ralph. He is willing to see the island destroyed in order to catch (and presumably murder) Ralph. In addition, Jack uses the fear of the "monster" to solidify his control over the younger kids.
- Roger is just plain mean. He is not described as being nice. We get the impression that Roger is capable of doing some really wicked things. While Jack is willing to be awful, Roger appears to enjoy being awful (or is at least grotesquely amoral.) Civilization does not operate on this principal. Everyone in a civilized society must share a similar moral code in order to function (one that Roger doesn't seem to have.)
- The Lord of the Flies is a rotting pig head on a stick. It is the very symbol of decay. It is the result of Jack's savagery. It is swarming with flies that enjoy it's rottenness. It tells Simon that man's nature is essentially bad, that the badness and cruelness is coming from inside of the children and therefore cannot be escaped from. His supposition that mankind is evil, again, is contrary to the types of ideas that make civilization function.
Hope that helps!