One of the themes that emerges showing the darker side of human nature is the desire to preserve oneself while others suffer and perish. Consider the plight of Moishe the Beadle, a deeply spiritual man who is forced into exile along with many other foreign Jews in Sighet. At first, the reaction in the town is of grief. They watch their former neighbors being loaded into cattle trains by Hungarian police and "[weep] bitterly." And soon, the deportees are completely forgotten. The people of Sighet create stories that their former neighbors and friends are living a satisfied life in their new "home," and this allows them to go on with their own lives without the burden of sadness or guilt. The winds of "reassurance blew through [their] houses" as they continue with their own plans and fairly normal lives.
When Moishe returns with stories of babies being thrown into the air and used as targets and of the deportees being forced to dig their own graves before presenting their necks for execution, the citizens of Sighet don't believe him. This directly contradicts the alternate and more pleasant reality that they have created in order to move on with their own lives, free of guilt or regret. They label Moishe a madman, and he begs them to listen in order to save themselves. But to listen to Moishe would be to admit that they might face this same horrible outcome.
Therefore, the people force Moishe to live as an outcast because of the truth he presents to them. In order to (temporarily) continue with a sense of normalcy, they cannot listen to the claims Moishe makes, which have a high likelihood of truth, based on all evidence that the people of Sighet are presented with.
This is one example of the need of people to preserve themselves in the midst of others suffering, but others will follow. Although there are moments of goodness where we also see the self-sacrifice of others (such as the French girl who gives Elie bread in the camp), the overwhelming evidence of human nature focuses on a need to preserve oneself, often at the expense of others.