I think that this is a great question because it opens up so much in way of thought and inquiry. When Fitzgerald uses the term "holocaust" with a lower case "h," the intent is to drive home the sense of destruction that the characters experience. The term is describing the landscape of the narrative: "It was after we started with Gatsby toward the house that the gardener saw Wilson's body a little way off in the grass, and the holocaust was complete." The death that is witnessed in terms of Gatsby and George Wilson, are holocausts in the sense of total destruction because they represent something more. The lifestyle of people like Tom and Daisy can no longer be seen as harmless and light. Rather, they bring about real pain and suffering to others. The entire social setting where so many are fraudulent and hypocritical can no longer be seen as devoid of consequences. The dead bodies stand as proof of this. In this use of the term "holocaust," destruction is evident in the physicality of death. However, it is also symbolic of the death of an attitude of lightness and triviality, a belief that there are no consequences to one's actions. It is a holocaust of beliefs that suggest individuals can escape from their reality. The dead bodies of both Wilsons and Gatsby stands as proof of this holocaust of belief.
Schlink's focus is the Holocaust. The upper case "H" reflects the historical reality where millions of Jewish people and millions of others were systematically removed from the Earth. Schlink's effect in this use is to show how the historical reality was caused by individuals. Hanna's role as a guard in the camps is the primary focus. Through utilizing the historical pain of the Holocaust, Schlink is able to display how individual actions are responsible bear consequences. Hanna must accept that her complicity with the Nazi machinery of death was a conscious choice. As Hanna learns to read, she absorbs the narratives of survivors, people she had a role in condemning to die, but lived. She understands why she bears responsibility and her death is an acknowledgement of that reality. The use of the historical context of the Holocaust is a deliberate means to display how human beings must bear responsibility for their actions. Even in the wide swaths cut by history, human beings are responsible for their actions.
In both works, the use of "holocaust" reveals how human actions can bring about pain and suffering. In both works, the effect of a "holocaust" is both historical and personal sense of destruction. Both historical and personal destruction is the net effect of the plot in each book. The thematic development of both works is one where there is destruction and pain. The use of the "Holocaust" and a "holocaust" reveals how political cruelty and personal hurt can be very similar to one another. The betrayal and hurt in one is mirrored by the disloyalty and suffering in another.