Who else thought Roy was going to be

absolutely. When they both eat in the beginning and he tells her that he was in the war and was in the military service but did not say on which side he totally spoiled that. At this time I thought, ok he was a Nazi and she is some jewish lady who wants revenge - in parts it was right so...

The movie was still a variation of the tired 'Jews hunting Nazis' trope.

The writers probably thought Helen being a persecuted Jew and Ian a Nazi was too cliche.

So big brain time - let's make Helen's dad a persecuted Nazi resistor and Ian a Nazi informant. Now that is totally different and completely new!