Was it necessary for the characters to do such terrifying acts in Lord of the Flies?

It was necessary if Golding was to prove his point. Golding calls hatred human nature. The children de-evolve from British school-boys (military to savages. Hatred is part of this progression. Jack, in particular, feels an intense hatred for Ralph. It is not so much Ralph but what he thinks Ralph can take away from him. Jack craves power. When Ralph is voted chief, Jack is mortified. Jack's "hate on" for Ralph stems from his own insecurities. This is contagious. Most of the boys internalize Jack's feelings. They do not Know why they have become snarling savages but hate is buried somewhere in there. Perhaps the most telling point in the novel is when Ralph asks Jack, "why do you hate me?" Jack cannot answer because he doesn't know. The killing of Simon, Golding's Christ figure, becomes a metaphor for fallen man. Golding believes the darkness of man's heart is inherit within people. When left to their natural impulses, people will inflict horrors on each other.