What horrible realization does Elie come to about Rabbi Eliahou and his son, and what is Elie's response to this?

In chapter 6, the Jewish prisoners are forced to evacuate Buna as the Russians quickly gain ground. Elie and the other inmates march in the middle of a snowy night as Nazi SS officers threaten to shoot anybody who stops moving. Elie compares the Jewish prisoners to automatons as they continuously march in the snow. Elie watches as several prisoners can no longer keep up the pace and are trampled by the moving pack of inmates.
When the Jewish prisoners finally arrive at Gleiwitz, Rabbi Eliahu asks Elie if he has seen his son. Rabbi Eliahu proceeds to tell Elie that during the journey, he could not keep pace with the other inmates and fell to the rear of the column. The rabbi tells Elie that his son didn't notice him losing ground and continued to march at a fast pace. After Elie tells Rabbi Eliahu that he has not seen his son, Elie remembers that Rabbi Eliahu's son had noticed his father losing ground during the march and purposely left him behind. Elie then thinks to himself that Rabbi Eliahu's son may have wanted his father to die in order to increase his own chances of survival, which is a horrible realization. Elie is glad that he forgot to tell the rabbi that he had seen his son and prays to never think or act like Rabbi Eliahu's selfish son.