Golding uses the two contrasting adjectives "harmless and horrible" to convey Simon's mixed feelings upon discovering the truth behind the beast on the mountain. Simon realizes that the beast is really harmless, because what Samneric saw in the dark was not truly a monster, but rather the rotting corpse of an entangled parachuter. At the same time, he also finds the true form of the beast to be "horrible," because it is a grotesquely rotting human corpse. When the "wind blew again, and the figure lifted,, bowed, and breathed foully at him[,] Simon knelt on all fours and was sick till his stomach was empty" (146). the appearance and smell of the beast is revolting; although it has no power to harm Simon, the boy's immediate response is one of revulsion to the point of vomiting. Discovering the human side to the beast is 'horrifying' in another way as well, because it forces Simon to acknowledge just how close the boys were to being rescued, again; if only the parachutist could have landed safely, the boys could have had their much longed for adult presence on the island, which would truly have been a huge comfort and help to all.
Simon surely feels the full thrust of the irony in his discovery and grimly appreciates how truly horrid (and 'horrifying') the moment is in finding the dead man, who could have been their salvation, but turned into the boys' worst fear. Simon's discovery about the beast and Golding's use of the dissimilar adjectives, "harmless and horrible" underscore a pertinent theme of the novel, appearance versus reality.