It is important to consider that William Golding wrote his novel, Lord of the Flies, as a reaction against the Victorian optimisim of Ballantyne's Coral Island in the wake of having served in a world war. For, after experiencing the horrors of war, Golding perceived an innate propensity in man for violence and savagery. And, in Lord of the Flies, once the constraints of society are removed through the passage of time, Jack becomes easily capable of the "evil that mean do," as Shakespeare's Marc Antony reflects on the deeds of the conspirators against Julius Caesar.
Golding and Shakespeare and a myriad of other authors address what the verses from the Book of Genesis [6:5] attest,
...the wickedness of man was great on the earth, and that every intention in the thoughts of his heart were evil intentionally.
Thus, because of this inherent evil, it is not long before Roger's arm, once constrained by "a society...in ruins" soon launches granite boulders onto Piggy after the other boys with spears and blows kill Simon. With man in general's propensity for wickedness, it is quite facile for Jack, then, to descend into savagery; for, after all, it is "the evil that men do." And, with the lack of society's restraints and the paint disguising him, Jack gives full vent to the primordial urges within him.
A thesis statement that incorporates the concept of man's inherent evil, "the evil that men do," emerging out of the license afforded it because of the absence of society may, then, be appropriate for the topic in question.