I agree that it has more to say about choices. Not all of the children in the book are evil. But we have evolved over millions of years in which the tendency to what we refer to as "evil" was a requirement for survival. It's easy for us to pass judgement today on what is good and evil, but in a natural state our inherited primal instincts will win out. Man first domesticated animals, it is believed, about 80,000 years ago. It took another 78,000 years before he perfected the viaduct. And only about 2000 years later to put a man on the moon. Our technology has grown exponentially and we tend to reflect this technological progress parallel with moral progress .... but this is not true. We are, to all extent and purposes still Cro-magnon man and it would take little in the way of a natural, or man made disaster which limited resources for life necessities to see a barbaric struggle for survival which would negate all moral considerations.
I agree that this is a difficult book to come to grips with, and much like Steinbeck can be difficult to read; but I think Golding is correct in assuming that the majority of people, when given a choice, in extremis, to aid their fellow citizens or themselves will choose the latter. I have seen examples of this in my own life. Not nice to witness but true. Of course there are exceptions to this rule, but they are so rare that we have invented special terms for such people .... "heroes" and "saints".