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

Elie remembers seeing Rabbi Eliahou's son during the run through the snow from Buna to Gleiwitz. Elie remembered seeing his son run up ahead and continue running without looking back.

He knows that the son deliberately ran ahead of his father to avoid being associated with him or near him at the rear of the line. Rabbi Eliahou's son chose "life" over family loyalty.  Elie sees Rabbi Eliahou's son as fickle and disloyal.

Elie promises himself that he will not behave toward his father like Rabbi Eliahou's son even if he is tempted. He does think of the extra rations he might be getting if he took from his father, but he puts this out of his mind and prays to God to give him strength and courage.

Elie proves himself to be true and loyal to the very end of his father's life.