In his allegory about human nature, Golding recognizes that the atavistic evil of man is not erased by civilization, it is merely conditioned to be somewhat controlled. Once the facade of civilization is removed and other masks are donned, the intrinsic evil of man emerges in the school boys stranded on an island away from the influence of adults and the "civilized" mores of British society.
"The evil that men do" [Julius Caesar] is worse than the actions of any animal. Few animals kill their own kind as humans do.
Golding is not saying humans are animals. He is, however, attempting to demonstrate his belief that man is born with an inherent sin nature which, without the restraints of civilization and the conformities of law, will culminate in savagery.
Rather than animals, then, it seems Golding believes we all have the capability for savagery--something quite different than being animals. Savage is the term he uses for the boys in the last chapter or two of the novel, and it implies a loss of self-restraint more than a loss of humanity.
The central opposition in the novel is that between the forces of civilisation and savagery, or order and chaos. Golding explores the competing instincts that dwell within us all: to live by rules, obey morals and act for the greater good of society, and then the opposite side, which is the desire to dominate, enforce one's will and act immediately to gratify desires.
This conflict is explored throughout the novel through examining the boys' gradual slide into lawlessness as they adapt to life in a barbaric jungle away from the normal controls on their behaviour (law, parents, school etc). These two forces are represented by the two characters Ralph (civilisation) and Jack (savagery).
Golding's conclusion is that the instinct for savagery wins out in the end. It is far more primal and fundamental to us than the instinct of civilisation, which he sees as a result of social conditioning rather than any moral goodness within humanity. We can see this through the example of the boys: when left to their own devices without any external forces of control, the instincts for savagery win out, even in the defender of civilisation, Ralph. The concept of the innate evil within all of us is central to understanding this novel, and is symbolised by the beast and the sow's head on the stake.
i don't think golding is trying to say all humans are animals, i completely agree what 'accessteacher' wrote, but i believe, if you want it simply, that Golding tries to convey that we all hold the capacity for evil, or as he writes 'the darkness of man's heart', just as some say everyone has the capacity for good.