It would be pretty cool if Kisame actully betray itachi and they get in* huge fight itachi would win like i said before with tsukoyomi(mangekyo sharingan)
itachi would defeat kisame easily. even kisame himself knows that.
only one person who can kill itachi easily i forgott his name but the inmortal member of akatski the one who killed sarutobi. hi cant die so the mongekyou sharigan has no effect wats so ever. the amateratsu burns everything but him.
the "immortal" members are Kakuzu, Hidan, Madara/Tobi (the reason I didn't mention Orochimaru is because he's dead and I don't consider him "immortal"). All but Madara/Tobi have ways of losining their immortality, but we don't technically know that Madara is immortal cuz he just fazes attacks through him or, sometimes, "erases himself from existence". Then again, him living for what...100 years should be proof enough, but (in the manga) Madara says that it's because of his "incredibly powerful chakra". So take ur pick of the reasons or believe what u want 2.