According to the Christian doctrine of original sin, humankind has been inherently sinful ever since our first ancestors, Adam and Eve, defied God by eating the forbidden fruit from the Tree of Knowledge.
As a result of their transgression, each and every human being since then has been born into sin. Even newborn babies are deemed sinful according to this teaching, despite the fact that they haven't yet had the chance to do anything wrong. This is because they have inherited the sin originally committed by Adam and Eve.
Golding vividly illustrates the doctrine of original sin in Lord of the Flies. Sin is presented as something lurking deep within the soul, something that is always there, waiting to show itself, irrespective of what kind of life we lead. Alone among the boys, Simon understands this. Unfortunately, this insight avails him nothing, as he ends up being brutally killed by the others in a savage frenzy.
The savage behavior of most of the boys throughout the story is a manifestation of sin. To be sure, the boys still would have been sinful according to the doctrine of original sin, even if they'd behaved like boy scouts the whole time. But their revolting behavior nonetheless illustrates just what was lurking inside them—and, indeed all of us, if you believe in the doctrine—all along.