Noah's statement said everything there was to say. Artamo didn't mind discussing the nature of his world or the cultivation journey, but arguing with Noah was beyond his interests. He couldn't bring peace to that conversation.
On the other hand, Noah had already made up his mind. He had to destroy another separate dimension to generate the key needed to reach the ninth floor. Artamo's ideals went against what Noah was trying to achieve, and his world was impossible to corrupt with ambition. The expert had to die so that those interested in the final battle could advance.
Noah activated his techniques without restraining the flow of potential. Artamo was a powerful liquid stage cultivator. His world didn't reach the same heights as Caesar, but it remained strong enough to require Noah's full power.
