In this "observation," Death notices two train guards and two grave diggers. One called the shots and the other did what he was told. Death asks, what if there are more "others"; that is, what if there are more people who simply do as they are told? Then, we have the description of the grave diggers:
"So hard getting through all the ice," and so forth. One of them couldn't have been more than fourteen. An apprentice.
This description seems to suggest 'what if there are a lot more other apprentices.' This small scene is a microcosm for the larger context of the Holocaust. The grave diggers are just doing their jobs but Death sees this in a similar way that one might see the Nazi soldiers. Like the apprentice who simply does as he is told and digs the grave dispassionately, what if there are others who mindlessly follow directions, mindlessly follow a leader. It seems likely that Death is using this scene to ask the question 'what if there are a lot more than one, many people, who do as they're told. In the larger context of the Holocaust, one of them called the shots (Hitler) and the "other" (a lot more than one) did what he/they were told. The grave digger was just doing his job, but Death could be implying that on a larger scale, the mindless following of a leader can be dangerous.