Kisame's loyalty is one of the more fascinating threads in the series for me, precisely because it's so different from someone like Itachi's. Itachi had this profound, tragic, clan-level ideology. Kisame's motivations feel more grounded in a warrior's ethos. He found a group, or more specifically, a person, who validated his entire worldview. He wasn't just a monster with a sword there; he was a trusted companion and a crucial part of a grand, world-altering plan.
His bond with Itachi is central. Think about it: the Akatsuki worked in pairs, but not all pairs had that depth. Kisame respected Itachi's strength and his calm, unshakeable resolve. Itachi saw Kisame not as a beast, but as a capable and intelligent shinobi. In a world that had only ever used him as a living weapon, that recognition was probably everything. His loyalty to the Akatsuki's dream of a 'true world' feels like an extension of his loyalty to the one person who treated him as a whole person.
In the end, he chose to die by his own hand to protect the group's secrets, a truly samurai-esque act. It wasn't blind fanaticism; it was the final affirmation of his chosen path and his partnership. That sense of belonging to something bigger than himself, especially after the disillusionment with his village, is what fueled him right to the very last panel.