Black Zetsu was a henchman who did Kaguya's bidding, he was the one who rewrote what was on the tablet, but it was Kaguya's intentions to be revived. It doesn't appear that way when Kaguya was revived though. Black zetsu seemed to be the mastermind in control of Kaguya, while Kaguya was just the muscle.
I blame Kaguya for being such a bad villain, to the point where her henchmen looks like the mastermind.