Good point about the Weimar Republic as an example of failure mode of democracy. I'm not sure whether it's germaine that part of the failure was it ceasing to be a democracy. Any other examples?
What is a failure mode? Are you seeking examples of bad outcomes and bad behavior in democracies, or something more specific?
A sidetrack: Are there any sound generalizations about differences between communist countries which had genocide, and those which didn't?
What are some examples of communist countries that have not engaged in mass murder? In Cuba and Nepal the death tolls haven't been so dramatic by Cambodian standards. Are there other tame examples?
Most of Eastern Europe, I think.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.