I think it's important to remember that the novel was written just after World War II when people were still reeling from the stories of Nazi atrocities that came out after the war. Six million Jews were dead, hundred of thousands displaced and the war all ended with a bomb that could now threatened all of humanity. Put in that context, I believe Golding's view of humanity is realistic---frighteningly so. If we look at the current world with it's terrorism, war, suffering, hunger and inhumanity, I think we can see that Golding had a point. There is an evil in all men. That evil must be controlled by good government and men themselves. The first step is to recognize man's capacity for evil, and then take steps to try to overcome it. Ignoring or placating it only leads to more evil, as Ralph discovers.
The novel itself isn't entirely realistic: Piggy's glasses, for example, if they were the prescription of the time for the myopia he suffers from, certainly wouldn't be able to light a fire in the way they do in the novel - it's also hugely unlikely that a conch found on a beach would sound like that. Come to mention it, it's also quite unlikely that so many boys (do they ever actually mention any dead?) would survive a plane crash.
You'll notice though, that the conch and the spectacles both function explicitly as symbols: the conch representing democracy, equality and civilised behaviour, the glasses clear-sightedness and wisdom (of course, they become gradually destroyed as Jack and his gang rise to prominence on the island).
I'd argue that "Lord of the Flies" is a fable, rather like Aesop's Fables, a deliberately heightened and neatened story, packed full of symbols to read, and judged carefully to deliver its message: its message being that, I think, every man's heart contains darkness (slightly paraphrased from the final page of the novel).
Golding's view of human nature is extremely pessimistic: do we really like to think that this is naturally how we evolve - toward war, towards violence, towards the murder of Piggy and Simon? I suppose not. But could anyone really, truly say that they don't think the behaviour of those boys is plausible? I don't think so.
Golding's presentation isn't wholly realistic: but I fear his message is.
The view of human nature that Golding presents is that people are innately savage, or evil, and that only the constraints of society keep people from exhibiting full savagery. In the book, Golding removes the group of boys from society and gives them the opportunity on the deserted island to start fresh. The society from which they were fleeing was killing itself with warfare. On the island, the boys could have begun a happy, peaceful existence. That's not what happens however. One boy, wanting power, manipulates and coerces other boys to follow him and overthrow the elected leader. The boys begin turning on each other, hunting and killing people as if they were animals. This view of human nature is certainly seen throughout the world and almost anytime a group of people gather, one will emerge as a leader. What is unrealistic, however, is the totality with which Golding presents the situation in the book. His characters are exaggerated representations of types of people. Seldom would there be a gathering with so many extreme examples of various types of people. Jack is determined and savage. Ralph is good, but clueless as to how to rule. Piggy is wise, but ineffectual. Simon is philosophical and understanding but dies before he can share his understanding. Golding needed these extremes to illustrate his point though. Man is capable of savagery, but he is also capable of peace and order.