Itachi didn't know Obito was Obito?
Most of the Uchiha members only have the original Sharingan so they aren't really hard to kill. Even looking at Obito before you say he becomes Tobidara or whatever, he is ok strong and Zetsu makes him even more powerful.
But as we saw, Obito is really powerful and of course, he was able to kill the Uchiha members. He had Wood Style, Kamui, and Zetsu strength. So it was pretty easy for him to kill the Uchiha members.
Tobi is Zetsu, but Obito was called Tobi for a little bit.