It is unlikely that Sai's mission is to kill Naruto. Unless he is at Sannin level, there is no way he can fight a Jinchuuriki and disciple of Jiraiya, a disciple of Hokage and an elite jounin level ANBU all by himself. Then Sai's mission must have something to do with the person(s) they are going to meet at the Heaven and Earth Bridge. In other words, Danzou knows who are waiting for them at the bridge. How can it be possible when even Tsunade, the Godaime hokage, doesn't know it? It is possible if Danzou has a bond with Orochimaru. Danzou might have already told Orochimaru about the spy and the meeting. Orochimaru wants to kill the spy. He also wants to kill Naruto to ruin Akatsuki's plan. So he and Sasuke will definitely turn up at the bridge. But what is the benefit Danzou will get from Orochimaru in return? Orochimaru might have promised to make him the Rokudaime Hokage. However, don't forget that Danzou was a Hokage candidate back in Sandaime's time. He must have got some wits in his head. He may have very strong desire to become a Hokage but he won't be that stupid to believe in the treacherous Orochimaru. Danzou might have pretended to take the offer but he is actually up to something else. Danzou is a "hawk". The difference between a "pigeon" and a "hawk" is that a "pigeon" will try to make the traitor back into a good person while a "hawk" will kill him straight away. Danzou probably sees Sasuke as a threat. He knows how much danger Konoha will face if Orochimaru ever gets the Sasuke's body. In order to ensure the future of the village(Sai's sempai said this is Danzou's will), Danzou orders Sai to kill Sasuke in the mission. A bit speculation on Sai's identity: I see some hatred in Sai's eyes as he read about the mission. I think it is possible that Sai is yet another survivor of Uchiha clan. He hates Itachi because he killed his little brother. Even Sakura herself commented on the resemblance between Sai and Sasuke. The fact that Sai has no family seems to support this speculation as well.