He probably killed another Kage not the prior one but maybe the dude who summoned the Clam that produces a genjutsu, or maybe the one before him.
Maybe a misconception? Perhaps Tobi let it happen since it's pretty obvious he was controlling Yagura by that time.
Word must have spread that Zabuza killed Yagura, when in reality Tobi let it happen since Yagura was technically dead already.
Incorrect, the Second Mizukage and Mu killed each other in battle.
The First was most likely dead years before Zabuza's birth, and Mei during the five kage summit talked a little about Yagura and how she lamented at his passing.
Is there anything known about the 3rd mizukage?
I thought Kisame killed the Misukage?
tobi or tobidara|???Tobi was the one controlling Yagura, so i doubt he killed him unless Tobi let it happen for some reason