I thought the same thing initially....and I in no way, condone those horrible and graphic images.
However, upon my second viewing this afternoon (eight years later) I saw what the producers were trying to illustrate....that ALL war, wherever, and whenever it is happening, always results in the death and disfigurement of otherwise innocent children.....and this was the premise behind the children in this film becoming the 'monsters' and 'murderers', that usually lies at the feet of the adults.
I understand the film a bit better with a second viewing.....although it has taken me so many years to get around to it.
After my second viewing, and re evaluating the beginning as well, I think the kids on the island were almost like a bacteria. For whatever reason they started acting like one organism. The group as a whole understood that the greatest threat to their survival was adults, and they changed to survive almost like how a bacteria might become resistant to an antibiotic.