There are no overarching bad guys. I mean ok in Arena it was Jagar Tharn. In DF, there was...no bad guy at all, just politics. Eadwyre was the worst of the lot, the treacherous, double-crossing jerk. Morrowind? Well, Dagoth Ur did have a point, and maybe Almalexia could be counted as the "true" bad girl of that one, but then again, the gambit Azura pulled - and she's not even one of the "evil" daedra lords!
So yeah it's always someone different.
So yeah it's always someone different.