I believe the various German monarchs were all deposed in the November 1918 revolution. I'm taking a Weimar history class right now, and basically all that was said on this was that the German defeat in World War One led to the German naval mutinies and eventually a popular uprising that overthrew the German Empire. The only thing that those proclaiming a new government seem to have been able to agree on is that the aristocracy would not take part.

FYI, your first two sentences are pretty redundant as it's the very definition an empire that it's made up from monarchies and an emperor (kaiser, czar, etc) is a king of kings.