Is not about people being sensitive, you have to keep in mind that your character from the future would probably not know what Gestapo or third reich was. I doubt people of Sirius would teach Earth history of 1900's. They would most likely focus on their own history, like the early days of colonial expansion, and would probably go as far back as the start of the Alliance-Coalition war.
You need to keep in mind people from Sirius do not identify themselves as "German" or "British" or whatever. They see themselves as Kusarian, or Libertonian, and would not know much about early Earth.
So calling someone gestapo or third reich is very OORP.
At least that's how I see it.