Before I interpret the movie On the Waterfront, it looks like I have to explain one of the pivotal events as a common trope:
Mob-connected union boss Johnny Friendly (Lee J. Cobb) gloats about his iron-fisted control of the waterfront. The police and the Waterfront Crime Commission know that Friendly is behind a number of murders, but witnesses play “D and D” (“deaf and dumb”), accepting their subservient position rather than risking the danger and shame of informing.
Terry Malloy (Marlon Brando) is a dockworker whose brother Charley “the Gent” (Rod Steiger) is Friendly’s right-hand man…Terry is used to coax Joey Doyle (Ben Wagner), a popular dockworker, into an ambush, preventing Joey from testifying against Friendly before the Crime Commission. Terry assumed that Friendly’s enforcers were only going to “lean” on Joey to pressure him into silence, and is surprised when Joey is killed.
This serves as Terry’s wake-up call. Joey’s death represents the loss of Terry’s naive, fantastical worldview about the culture he operates within. Later on, the loss of the worldview is used by some outsider technocrats to turn Terry on his erstwhile benefactors, because he’s no longer able to rationalize his pent-up resentment about how cooperating with the culture had also been hurting him, personally.
The best example of this “innocence lost” meme can be seen in the contrast of the two main characters of Final Fantasy Tactics, Ramza and Delita. Their sisters, Alma and Teta, represent their respective idealism about the medieval caste system. The pivotal moment of their relationship is the death of Teta, directly caused by her low station, which drives Delita to exploit the chaos of war to ascend the caste hierarchy and become king. In contrast, Ramza spends the majority of the game protecting his sister and, later, chasing after her kidnappers. In contrast to Delita’s response to the war, Ramza always takes the hard way, doing whatever he can to ameliorate the savagery of the chaos around him. Delita loses his idealism and gains the world, while Ramza preserves his idealism and loses everything else: his rank, his family, and his historical reputation, becoming a criminal and heretic constantly on the run from enemies (eventually almost everyone he meets).
Typically the representation of “innocence” is some form of innocent person, like a “divine child” (in the King/Magician/Lover/Warrior sense), a wife, a daughter, or some other sort of person who we feel ought to be protected from harm. Further study:
In the story of Cain and Abel the man murders his own ideal out of resentment, but in a typical “innocence lost” story it’s the monster that kills the ideal.