I'd give this 4 stars as a young adult book. Reading it as an adult however, I'd give it probably only 3. It was a quick read, where you want to keep turning the pages and I enjoyed it, but the symbolism was a bit over the top and the characters were underdeveloped. Piggy was probably the most developed but even his was minimal. Despite the lack of development, the book does still drive it's narrative and give some good food for thought. 
 There seems to be much analysis about this book that extrapolates how human nature is inherently savage without rules, civilization and order. However here, the savages had rules and order, they just had a more ruthless leader in Jack. They actually had less rules and order under Ralph than under Jack. Also, not everyone fell into savagery and some who did had regrets. Why? Were they defective humans? For me, this book is more about how easily people are led by fear, the corrupting influence of mobs and the human survival (fight or flight) instinct. 
 In the context of World War II, you can't help but think of the evil acts Hitler led some previously decent people to under his brutal reign. It would seem that too many rules and too much "order" only protect nefarious rulers whereas not enough rules or order leads only to the rise of such rulers. I don't believe this is because we are inherently savage, but because most of us are weak and unable to stand up to public ridicule, torture, starvation etc... In order to survive, people end up justifying and/or ignoring behavior that they know to be wrong precisely because of our inherently good nature. What we have is more of a survival instinct than a savage one. Most would choose to fight or flee the evil if they knew they could defeat or escape it. This survival instinct under the stress of a savage ruler turns the "fight" against what might anger the savage ruler. That does not mean people are inherently savages. There is evil in this world manifested by evil people who don't have a conscious, but those are the defective humans, not the other way around.