Their data had been insufficient, Itachi admitted that. He wasn't one for denial -- not that he could have denied that particular unpleasant truth.
The Fourth had been a vastly superior seal expert than any of the ones who had performed the seals on the previous demon holders. Even Shuukaku and the Three Tails hadn't been as hard to extract, and contrary to the rest, they had live carriers too; but it hadn't been too complicated to unravel the seals, and the death of the carriers had, in the end, pulled the demons free. Itachi wasn't sure the seal expert of the Akatsuki would find a way to unbind the Kyuubi's. Basically, Kyuubi couldn't be taken out of Naruto-kun. At all. What it meant, was that Naruto was probably a lot closer to immortality than Orochimaru would ever get.
What it concretely meant, was that Itachi was fucked. Because the vessel and the fox were bleeding into each other at the edges, and he'd angered them both -- even before capturing them to lead them to be separated.
He'd angered them both, by attacking them, and hurting their teammates -- for putting Kakashi-san through Tsukiyomi again, and breaking that pink-haired kunoichi's legs, and, surprisingly, for traumatizing his brother. He'd angered them both, by being the reason Sasuke betrayed them for the Sound, fought them with the intent to kill. By being the indirect reason they ended up in the hospital, broken and bloody and beaten.
He'd angered them both, mostly, by daring to look like Sasuke.
The relationship between his little brother and Naruto-kun was apparently complex and of a mixed nature -- and frankly of no real interest to him -- but one thing it seemed to produce in abundance was enraged resentment.
Naruto was too squeamish to kill him in cold blood, but Kyuubi had no problem finding new and inventive ways to make him bleed. Ways that made Naruto forget exactly who he was yelling at, who he was hitting, who he was blaming for being a cold asshole and hurting his friends and abandoning him.
Ways that made Naruto forget who he was bruising. Biting. Kissing.