what's the writer's purpose in Lord of The Flies?

I think that Golding wanted to point out that humanity is flawed and we naturally gravitate to the darker side of the soul. The culmination of the plot in war and murder suggests that Golding's overarching hypothesis about humanity is pessimistic, that is, there are anarchic and brutal instincts in human nature. Ordered democracy or some other regime is necessary to contain these instincts.