People are not born inherently good because children do what they learn to do. That is kind of the whole point of parenting so that you teach your children the difference between good and bad. When a child is born they don't know anything they are a clean slate. Even if someone is born into a good environment and their parents teach them the difference between right and wrong and raise them to be good they could still end up bad.
The kids in the book where in a military school and obliviously had discipline and structure they where raised to be good. Jack went crazy and did a lot of bad stuff and he started going crazy and killing pigs. Jack ended up not caring about being civilized he wasn't the only one they were all responsible for the death of Simon because they beat him to death.
All of the boys in Lord of the Flies ended up turning into savages and painting their faces and dancing around like uncivilized people. Roger killed piggy he was part of the school group when they got to the island he was also civilized and wanted to order but he went savage too. Jack was part of the boys choir he was actually the leader of them but he still went bad.