What horrible realization did elie come to concerning rabbi eliahou and his son an what was his response to this?

Elie witnesses the rabbi desperatly looking for his son during the prisoners' horrible march to Gleiwitz. The Rabbi insists that his son would never leave him. Elie had seen his son march ahead when his father stumbled in the march. The Rabbi's son had forsaken his father. Elie notes that he is capable of doing the same which bothers him.