Good vs. evil in Lord of the Flies: Explain which one triumphs, giving three reasons why.

Consistent with his theme that humankind is innately depraved, Golding has evil triumph over good in Lord of the Flies. Even though he doesn't write the book so that all the boys die, or even so that all the "good" boys die, evil achieves the upper hand as evidenced by the symbolic loss of spirituality and reason through the deaths of Simon and Piggy; Jack's embracing of premeditated murder; and the betrayal of Samneric, the last boys loyal to Ralph.
The point at which evil begins winning in the novel is the murder of Simon by the mob. Simon, the sensitive boy and the Christ figure, represents spirituality, religion, and/or philosophy. Simon is the only one of the boys who understands "mankind's essential illness," and he dies while trying to bring them the good news about the beast--that it "was harmless and horrible." That not just Jack but also Ralph, Piggy, and Samneric try to re-write history to deny their culpability in his death gives evil the upper hand. The "good" boys, those who haven't joined Jack's tribe, then try to muddle along with reason alone, represented by Piggy, the most intelligent of the boys. He is first handicapped when Jack's tribe steals his glasses, then is deliberately murdered. Spirituality and reason are the best ways to keep human depravity in check; without them, evil quickly spreads and gains control, just as Jack's power quickly becomes complete after the deaths of Simon and Piggy.
When Jack embraces Roger's brutal killing of Piggy by saying, "See? See? That's what you'll get!" he fully gives himself over to evil. If seeing a murder committed in front of him by one of his boys in broad daylight can't shame him, he is beyond hope. That proves to be true, for he immediately plans to hunt and kill Ralph as if he were nothing more than a pig. This shows he has fully embraced evil as his way of life--and he will lead the rest of the boys down that same path.
When Samneric, who have been made part of Jack's tribe against their will, betray Ralph by divulging his hiding spot, even the faint hope that at least some boys will retain their integrity and fight against the darkness dies. Although Ralph imagines that they are "hating it" as they are hunting him with Jack, that means nothing. The fact is, they comply with his evil regime, succumbing to physical torture rather than holding onto what is right. Ralph cannot survive on his own when there is not a single boy to help him. Evil has won.
Although Golding allows a naval officer--a representative of a saner portion of humanity that one hopes is doing its best to fight against an evil regime in the war taking place in the greater world--to rescue the boys, it is something of a deus ex machina. Without the help of a "higher power" the boys would have killed Ralph first, then all would have perished in the fire or from starvation. Golding wraps up his fictional social experiment, showing that evil is powerful enough to triumph over good, and presenting a clear warning that all must be on guard to prevent that from happening in our world.