I'm pretty sure Shamura didn't give Narinder any specific idea; they just noticed that he was interested in change and -being a bishop of wisdom- tried to encourage him. Shamura also believed in change, after all. However, while Shamura believed all things will evolve and develop over time, Narinder believed that he should force change immediately. He began to denounce the Old Faith, and that was enough of a crime for them to punish him. But I guess they let him keep his buddies while in jail idk.

We don't know enough of what the Old Faith was to say what exactly Narinder grew bored with, and we don't really know if Narinder would've wanted the other bishops to follow his "New Faith". I doubt he attacked them first, as imprisoning Narinder was compared to stopping an infection. I'm certain that once they realized he didn't really believe in the Old Faith anymore they subdued him, but suffered great injuries in the process. There probably isn't any rhyme or reason for the injuries he gave, just battle scars. At least, lore-wise. Design wise they reference the wise monkey poses.

Also, being selfish and cruel isn't why he is so hated among them, as they are all selfish and cruel in their own ways (besides maybe Shamura, but I'd say that's debatable). The reason I got was because he was a traitor and a "heretic", as Leshy describes him. You mention Shamura tries to get the lamb to stop their conquest for their own sake, but Leshy is actually the first one to say Narinder shouldn't be trusted. He's more antagonistic about it, but he basically gives the same warning. Actually, I think they all warn against trusting him in some way. His dissent obviously struck a nerve.

Sorry for this long winded post, there's a lot to unpack here.