I'm guessing Tarnak locked Amuut away because he's always been the overly cautious type, while also being rather arrogant, and didn't trust someone that was as strong as him (or at least close to the same strength since he's at an S rank level.) You could argue it was for Amuut's protection, as the Iron body Monarch's descendant, but you don't normally imprison someone like this if you're just trying to protect them.