Not sure if it's reasonable to expect people to badmouth their current employer in an interview, especially when you don't know the result of the interview.
The last ex-Amazonian I interviewed managed to express his discomfort with Amazon without "badmouthing" anything or anyone. I was impressed with his diplomacy. We hired him.
So it becomes an interview of diplomacy essentially. Not really sure if that's ideal. I could see many false negatives of people who are technically strong, uncomfortable with their employer's ethics but still avoiding the topic so as not to say too much by mistake.