Disclaimer: I don't own Naruto.
Itachi was stuck. The limestone cave was impaling him from all directions – a result of his instinct to protect his younger brother before himself. Now, the only thing keeping him, well 'alive' was hardly the right word, was the Edo Tensei, but even that couldn't free him. He released the Susano'o protecting Sasuke, realising that Kabuto had no reason to harm him. A bit late, he scolded himself, realising that the unnecessary protection had left him vulnerable to a new tag, one that would override Shisui's genjutsu. The expected tag was produced and he inwardly cursed as he futilely attempted to free himself. If he was once again brought under the control of Kabuto then the Shinobi would lose, because no-one else would be able to find Kabuto, let alone make him release the jutsu. Kabuto was approaching steadily, seeing no need to rush – and rightly so. Itachi had no way to escape, no way to prevent the kunai being inserted into his mind and taking over his actions once again.
Black flames erupted in a circle around them, so intense the Ryu Sennin was forced to retreat slightly, and the limestone reverted back to a normal cave, retreating from Itachi's body and allowing the Edo Tensei to completely reform his body. He blinked, the reality of the situation setting in. Sasuke had saved him.
With a start, he realised that this was a first occurrence. In all his twenty-one years of life, and death, his younger brother had never saved him, had never had to save him. He kept his emotions firmly behind his usual mask, but inside he was in turmoil. His younger brother, the one he had nurtured and cared for, even if it was just from the side lines for most of his life, had finally surpassed him – had become the protector, not the protected. Itachi wasn't sure how to handle it. On the one hand, it was a reassurance; Sasuke could look after himself without Itachi, so he wouldn't have to worry when he disappeared with the Edo Tensei. On the other hand, his role as the older brother was to always protect his baby brother and now that that had been reversed he felt useless. There was nothing more that he could do for Sasuke, and that made him sad.
Then it happened again. This time the tag was thrown, and Sasuke had first tried to stop it with his own katana, but that was blocked, so he had used his Susano'o to protect him. A one-off Itachi could deal with, pretend it was a fluke so that he still felt needed, but at the same time be reassured that Sasuke could look after himself. A second time, more deliberate than just a shield of black flames around the two of them, was more than he could bear. It proved that it wasn't a fluke, that Sasuke really was actively protecting him, despite the fact that it should be the other way around.
It was at that point that he realised that it was time to let go. Sasuke didn't need him anymore, despite what he selfishly hoped, and that the best thing he could do, for everyone he cared about that was still living, was to end the Edo Tensei as soon as possible, and disappear. For one, he had to put aside his own wants, his own needs, and put those of Sasuke first. Sasuke needed a new barrier, a new target to surpass; that was something he wouldn't find until Itachi was gone for good.
His purpose now assured, he stood tall and prepared to fight until the bitter end. His final end.
Even though I read the manga, it wasn't until I watched the anime version of this fight that I realised that it was the first time Sasuke had ever protected Itachi in a fight and thus this short thing was born. Being an older sibling myself, I can somewhat imagine what it would be like to have my younger sibling turn the roles on their head, and be the protector and not the protected. For Itachi, who lived his whole life for the sake of protecting Sasuke, I thought that it was be a major shock to the system; a final reason to let go once and for all.
Thanks for reading!
Tsari