itachi never stopped being evil. if he truly wanted the safety of kyuubi's jinchuriki hed have stabbed kisame in the back with totsuka and trained naruto himself in some remote village then together, killed every last akatsuki. or at least not joined and tried to strengthen an evil tyrant who he formed an alliance with.
He would've made up another excuse to kisame if naruto did in deed come with them. Kisame seems to take itachi's words as absolute and rarely questions his decision
and what was he gonna do when kisame was seconds away from cutting narutos legs off? reveal his betrayal to the rest of akatsuki by kisame suspiciously disappearing on a mission? what if jiraiya's toad was just a few minutes too late in arriving there at the hotel?
but he knew that Naruto cares about Sasuke
he didnt figure that out till naruto showed concern at sasukes later beating
no, just no Kisame already said that Itachi would destroy him.
probably cos he thinks itachis ruthless and doesnt care about anyone whereas kisame would be hesitant to hurt his friend. plus he knows about itachis ms abilities. in an open space kisame pwns