Sunday was never in a position where his life was ever in danger

The party fights him as a final boss...how much more endangered does one's life need to be?

writing of future stories is more likely to be impacted by the player response to previous arcs, though...

Yeah, another indication of bad writing. Rather than writing the story set up by previous plot points, they're writing to suit player wants, which doesn't stay consistent to the pervious written material.