You remembered the story quite well, though some of the details are different. The avenger was a Romanian Jew called Eliahu Itzkovitz and the man who killed his family was a fellow Romanian (Romania was an ally of Nazi Germany and implemented its own policies of persecution and murder of its Jews) called Stanescu. Itzkovitz emigrated to Israel after the war where he learned from some fellow countrymen that Stanescu had joined the French Foreign Legion. Whereupon Itzkovitz does the same (first deserting from the Israeli Navy), tracks down Stanescu in Indochina and guns him down. The death is attributed to the Vietnamese enemy. After his discharge from the Legion, Itzkovitz returns to Israel to face a court martial for his desertion, but gets off lightly because of his motive in deserting (and probably also because he deserted in peacetime).

This story purportedly happened in 1954 and was first told in 1961 in Street Without Joy, a work on the French wars in Indochina by Bernard Fall, a professor in political sciences and international relations in the US, as well as a (war) journalist and writer. Fall is quite a colourful character himself. Originally from Austria, his Jewish family fled to France when Germany annexed Austria. At the age of 16 he joined the French Resistance after the Germans overran France, and later the French Army. Both his parents died during the war, his mother in Auschwitz. He later studied at numerous universities in Europe and the US. His particular focus was Indochina and he travelled to the region often during the fifties and sixties. He died stepping on a landmine in Vietnam in 1967.

Is this story true, you may well ask. On the one hand, Fall was a respected scholar. On the other hand, he does not cite any sources. The story was repeated almost verbatim in Charles Mercer's 1964 Legion of Strangers (which lists Street Without Joy in its bibliography) and by this means found its way into numerous other books on the French Foreign Legion. The trail seems to end without a definite answer.

Until, in 2019, a book appears called Eliahu Itzkovitz La Vengeance d'Un Enfant Juif (A Jewish Child's Revenge). Written by Gabriel Joshua Saada (and by ghostwriter Djemâa Chraiti), who claims to have been a friend of Itzkovitz and to have promised him to write down and publish his remarkable story only after his (Itzkovitz') death in 2015. Why after his death? According to Saada, because Eliahu had been sworn to secrecy in a sort of plea deal at his court martial. The sceptic in me says: dead men can't tell you to quit your bull- you know what I mean.

Is the basic story told by Fall true and this French sensational book a fraud trying to cash in on a dead man's tragic past? Or is Saada really the one friend Eliahu told all the details to? Or is the original Fall story a figment of someone's imagination as well?

Saada's book mentions seeing some French magazine and newspaper articles that reported the story at the time. One he names in particular is the huge headline "Eli le vengeur" (Eli the avenger) in Paris Match (sort of a French version of Life but with more celeb gossip). As an aside, this doesn't seem to square with the secrecy pledge at the court martial: what's the point if the story had been plastered all over the popular French press? A search of Gallica's (digital collection of the French National Library) newspaper and magazine archive for various permutations of the name Eli(ahu) Itzkovitz turned up nothing. But that's no proof in itself as they don't have Paris Match, for instance.

Will we ever know for sure?

Sources

Dorothy Fall. Bernard Fall: Memories of a Soldier-Scholar, 2006 (written by his widow)

Bernard Fall. Street Without Joy, 1961

Charles Mercer. Legion of Strangers, 1964

Gabriel Joshua Saada & Djemâa Chraïti. Eliahu Itzkovitz La vengeance d'un enfant juif, 2019