I know this is more of a chapter than an answer, but bear with me! If you would prefer a more concise version, just can stick to this first paragraph.
Everyone in Lord of the Flies is "bad", and, by extension, so is everyone on earth. People are inherently savage. That was Golding's message, having been appalled by wartime atrocities as well as war itself. Even Ralph, the protagonist, and Piggy, who seemed wise and good-natured, succumbed to barbarism by helping to kill Simon. Golding's novel symbolises the fact that quiet, subversive evil will survive, whereas any modicum of goodness will be destroyed.
The thing about Nazism that Golding found most horrendous was not that there was one bigot who believed in terrible things, but that other, normal people ended up following him. He believed that society was governed by survival of the fittest (think of Hitler's so-called "Aryan race"), meaning they were happy to discriminate against outsiders.
Excerpt from William Golding's essay Fable, given as a lecture in Calafornia, 1962. (I bolded the important text)
"Before the second world war I believed in perfectibility of social man; that a correct structure of society would produce goodwill; and that therefore you could remove all social ills by a by a reorganization of society. It is possible today that I believe something of the same again; but after war I did not because I was unable to. I had discovered what man could do to one another.
I am not talking of one man killing another with a gun, or dropping a bomb on him or blowing him up or torpedoing him. I am thinking of the vileness beyond all worlds that went on, year after year, in the totalitarian states. It is bad enough to say that so many Jews exterminated in this way and that, so many people liquidated, but there were things done during this period from which I still have to avert my mind lest I should be physically sick. They were not done by the headhunters of New Guinea, or by some primitive tribe in the Amazon. They were done, skilfully, coldly, by educated men, doctors, lawyers, by men with a tradition of civilization behind them, to beings of their own kind.
I would like to pass on; but I must say that anyone who moved through those years without understanding that man produces evil as a bee produces honey, must have been blind or wrong in the head. [...] It seemed to me that man's capacity for greed, his innate cruelty and selfishness was being hidden behind a kind of pair of political pants. I believed then, that man was sick - not exceptional man, but average man. I believed that the condition of man was to be a morally diseased creation and that the best job I could do at the time was to trace the connection between his diseased nature and the international mess he gets into.
[...] I decided to take the literary convention of boys on an island, only to make them real boys instead of paper cutouts with no life in them; and try to show how the shape of the society they evolved would be conditioned by their diseased, their fallen nature."
In this context, the people in the minority were regarded as weak. People selfishly followed the 'strongest' leader in order to 'win', which is why Ralph (who is democratic) is hunted down, and Piggy (his supporter, the smartest boy on the island), is killed. They ultimately lose out to Jack (who is needlessly corrupt) and Roger (Jack's sadistic helper). Desperate not to be hunted as outsiders, Samneric betray Ralph when tortured by Roger.
The child, as Piggy said, "what had a mark on his face", is wiped out almost instantly by the fire. His minor deficiency (a birthmark) costs him his life. As savagery flares, the killings become worse, starting with a sow (who even has piglets). Make of this what you will, but it's hard to miss at second glance:
"bleeding and mad [...] wedded to her in lust, excited [...] blood and terror [...] prodding with his spear [...] stabbing downward with his knife [..] leaning in with his whole weight [...] inch by inch [...] high-pitched scream [...] heavy and fulfilled upon her".
Jack's attitude morphs into an increasingly graphic allusion to Hitler. He laughs at his "reeking palms" having killed the pig, where in an earlier chapter he at least "grimaced distastefully".
Roger's evilness escalates much like Jack's. When Jack orders him to sharpen "the stick at both ends" to mount the head, he does so without question. He kicks the littluns' sandcastles and throws stones at Henry but aims to miss as a result of "the taboo of old life", but the littluns fear him already. When Jack discovers masks, Roger takes "liberated from shame and self-consciousness" to the next level, by levering down a rock to kill Piggy.
The rock struck Piggy a glancing blow from chin to knee [...] Piggy fell forty feet and landed on his back [...] his head opened and stuff came out and turned red. Piggy's arms and legs twitched a bit, like a pig's after it has been killed. Then the sea breathed again in a long, slow sigh, the water boiled white and pink over the rock; and when it went, sucking back again, the body of Piggy was gone.
Simon alone acted morally for its inherent value. In terms of Golding believing that mankind is all evil, Simon is the exception that proves the rule. He alone is consistently kind, regardless of the consequence - he gives Piggy meat, thereby braving Jack's anger, he tells Jack to suck his wound although Jack bullies him, and he picks food for the littluns who can't reach it. He fearlessly seeks the truth, and confronts the beast. When he understands that the beast resides in them, he takes the ultimate risk. His bravery backfires, much like resistance did during the era of Nazism. Golding seemed to believe that mankind was doomed.