What interests Melville in this tale is not so much the battle between good and evil but the presentation of innocence in the face of experience. In this sense, Billy is not presented as a character who is defined by his moral state of being good, but he is rather presented as a character who is extremely naive about the way that the world works. Note, for example, how is character is presented in Chapter Two by the narrator:
By his original constitution aided by the co-operating influences of his lot, Billy in many respects was little more than a sort of upright barbarian, much such perhaps as Adam presumably might have been ere the urbane Serpent wriggled himself into his company.
This, as the text shows, is a positive liability in a world where characters such as Claggart exist. Billy's innocence means that he is not cognisant of good and evil in order to be able to distinguish between these two moral states. The Biblical allusion to Adam cements the presentation of Billy as an innocent who because of his innocence is dangerously vulnerable to manipulation and exploitation by evil, and unfortunately, evil is shown to come in the form of Claggart, who leads Billy towards evil and directly causes his downfall.