While it is hard to say what they wanted to say (even my commented edition does not has any special remarks on it).

I would interpret it as a contraception part of the character in “The Experiment”, Fritz Geiger who was a Nazi by hearth.
Chained to the machine gun I would interpret it that he was forced to fight – no matter if he stood behind the ideology or not. Just like many people in Arkanar.

Please note that this is my interpretation, considering also the works “The Experiment” and “The boy from hell” (I don’t know the English title, in Russian its “parjen` is preispodnej”

Also note that this was one of the first novels with their ideas of extraterrestrial influence from a more advanced civilization, it could be that they just used the framework to bring the reader closer to the idea of the main story.