That's the only route he could take.
What was he going to tell Sasuke? That he, part 1 Kakashi, was going to make him stronger than Itachi/could teach him more than Orochimaru? lol, Nope. Itachi had already put Kakashi in a coma and Orochimaru made him shit himself. Sasuke knew better than that. Jiraiya was the only person alive in the village at that time capable of training someone to be equal to or more powerful than what Orochimaru could, and he had Naruto to deal with. Maybe if Kakashi could have made Sasuke see the future and seen what War Arc KAkashi ended up becoming, maybe he would have had some incentive to stay with him. But as it was in part 1? Nope.
And Kakashi didn't have a choice in whether or not to go. They'd be dead if he didn't. Shikamaru lost all intelligence he had and was going in to a fight where he only had intel on one of the two fighters abilities, and it was shown very, VERY early on how badly Kakuzu outclassed team 10 by himself, much less with Hidan's help.
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Kakashi puts his comrade's lives above his own, as he's stated and shown numerous times in this manga. He wasn't going to let Team 10 go die.