Why did the old man have to kill himself?

Boris the grandfather is an anti-semitic Jew. In the movie he kills himself in the because of his unbearable guilt over denouncing his Jewish heritage and raising his family as non-Jews.He basically hates himself for his hatred of his own people.

The Grandfather's name is Eli (pre-holocaust) and Alex (Post-Holocaust) - the same as his son and grandson (Alex the translator) - not Boris.

The Grandafather kills himself because of his guilt, but to a larger extent, because of the fact Jonathan sees Alex (translator) as a perpatrator because of his heritage - for example, the Grandfather pointing Herschal out as a Jew and to his death. Post-memory is a theory widely attributed to Everything is Illuminated that indicates, despite Alex (translator) and Jonathan's distance from being directly affected by the holocaust (by three generations), the trauma, guilt and/or victimisation is trickled down through the generations. The paradox of the marrying together of Alex (translator) and Jonathan manifests as perpatrator and victim / Ukrainian and Jew. The Grandfather saves the family from guilt and responsibility of being a perpatrator in the holocaust by committing suicide in order to cease the direct link of trickle-down responsibility. By doing so, the Grandfather offers a chance for Alex (translator) to be forgiven by Jonathan because of Alex's heritage of being a perpatrator to Jonathan and thus to Jews. This is in response to Jonathan's lack of forgiveness to Alex as shown through the final chapters. There's a reading that Jonathan does not reply to Alex and so does not forgive.