In Lord of the Flies and Heart of Darkness we watch as formerly civilized people fall into a state of violence, barbarism, and evil. In Lord of the Flies the boys become superstitious of the beast, split into fighting groups, and eventually even burn the island in their quarrels. In Heart of Darkness, a man of great civilization becomes a murderer and a savage.
Both novels explore the lurking dark side of human nature. The heart of darkness, as it is described in Conrad's title is something evil in our nature that lurks just below the surface. Without the tempering of society and with the stresses of danger we will often revert to our darker nature. The terror of Heart of Darkness isn't what the men find among the Africans in the Congo; it is the darkness they discover lurking within themselves, particularly in Kurtz. This darkness was there long before traveling to the Congo, hiding beneath the surface. The same darkness exists to varying extents within all of us, including the characters in Lord of the Flies.
Left to their own devices, the same darkness that we see in Conrad comes forward in Golding. The boys turn to savage tactics to survive and gain control of one another.