What horrible realization did Elie come to concerning Rabbi Eliahou and his son? How did Elie respond to this?

Elie realized that the Rabbi's son ran ahead of his father on purpose.... that he wanted to increase his own chances for survival.

Then I remembered something else: his son had seen him losing ground, limping staggering back to the rear of the column. He had seen him. And he had continued to run on in front, letting the distance between them grow greater. A terrible thought loomed up in my mind: he had wanted to get rid of his father! He had felt that his father was growing weak, he had believed that the end was near and had sought this separation in order to get rid of the burden, to free himself from so encumbrance which could lessen his own chances of survival.

Eliezer prayed to God that he might never do the same.

And, in spite of myself, a prayer rose in my heart, to that God in whom I no longer believed. My God, Lord of the Universe, give me strength never to do what Rabbi Eliahou's son has done.