Volksdeutsch were valued for their German racial "purity" and ethnicity, loyalty to Germany, and ability to speak eastern European languages of the victims, e.g. Ukrainian, Polish, Russian.
The Volksdeutsch' ability to speak German, made them valuable especially because Yiddish, which has many German roots words, could be understood by the Jewish victims, in particular. The Volksdeutsch could understand Yiddish spoken in Eastern Europe, including Ukraine, where many of them were recruited.
When the Volksdeutsch guards needed to communicate with the Jewish "kapos," who collaborated with them, the ability to speak German was, because of its similarity to Yiddish, and the Volksdeitsch familiarity with Yiddish, especially important for day-to-day operations at the murder camps.
Demjanjuk was an ethnic Ukrainian--not a Volksdeutch--therefor, that makes him less likely to have been a so-called "Ukrainian Guard."
In any case, there are no witnesses for the prosecution who can remember anyone like John Demjanjuk being at Sobibor, which in itself, says something.
As horrific as the crimes at Sobibor were, it should also be remembered that Sobibor is not on trial, but John Demjanjuk is, an so far there has been no valuable evidence that would incriminate him.
Also, John Demjanjukwas a Red Army soldier taken prisoner by the German Nazis and subjected to a fate as bad or worse than the Jews. It is a well documented fact that millions of Red Army prisoners were murdered by the Nazi Germans by extreme privation in open air concentration camps, as well as at death factories like Auwschwitz. Demjanjuk had been a prisoner at some of those camps and therefore is no less a victim that any of the other victims of Nazi Germany.
One can only wonder at the then mental state of mind of Nazi Germany's victims like John Demjanjuk, during those most inhumane an cruel ordeals.
Didn't Nazi Germany's psychopathic inhumanity and callousness make their victims mentally ill, and if so, does this diminish responsibility?