What makes this movie so frightening without spectacular, graphic violence or overly righteous hand wringing is demonstrating that the architects of even the most extreme forms of evil can be quite human and even likable. They may honestly view their workaday world of mass murder as simply a day at the office. The film "The Wansee Conference" makes the point even more effectively but isn't nearly as artful, since it is a straight, documentary style re-enactment of a specific event.
The only thing that hurts the movie is the pairing of "Klink" (Werner Klemperer) as Eichmann and "Schultz" (John Banner) as his chief lieutenant, Hess (no relation to Rudolph Hesse). Of course, this was an unknown dynamic when the film was made since it predates Hogan's Heroes by 2 or 3 years. The actors are actually perfectly matched for their roles and play them well, but their popular identification as the "funny" Nazis of Hogan's Heroes gives the film an unintentional comic slant at times. In fact, there is a scene where Eichmann is having dinner with Hess and his wife and kids. A seemingly normal family scene with dad and his boss discussing business----in this case, how efficient Zyklon B gas is for human extermination. The Hess character replies, (I kid you not) in his best Sgt Schultz voice and manner, that "the best thing about it is that you hear NOTHING you see NOTHING..." As an interesting aside, Werner Klemperer, John Banner, and Hogan's Heroes General Burkhalter (Leon Askin, who is not in this movie) were all Jewish refugees from Nazism who fled their homes in Germany and Austria. Also of note is the fact that "Klink" served in the U. S. Army in WWII; ironically, in the Pacific. Askin and Banner both served in the US Army Air Corps. The former, though only a sergeant, had a plum job teaching European etiquette and social practices to U. S. military officers. "Schultz" was a supply sergeant.
This low budget film effectively communicates the fact that the most fiendish enterprise can readily disguise itself as business as usual.