If you're suggesting that humans can't be that terrible, I recommend that you do not open this link (It is a link to a BBC article but I feel compelled to give a trigger warning since it is genuinely that disturbing): https://news.ycombinator.com/item?id=45942652 History is rife with similar examples; those who still believe there is any inherent goodness in human nature should acquaint themselves with them.
Golding is right on the money. Humans are merely animals and will behave no better than animals given the right circumstances.
> suggesting that humans can't be that terrible
No, he's suggesting that children usually aren't that terrible. That a real scenario of ~50 unsupervised children (or adults), 99 out of 100 times, wouldn't play out that way. That something is possible does not mean it is the norm, and only those that can't grasp numbers (such as English majors) think otherwise. With such significant caveats, can one really say that the novel is about human nature in general?