The detail that best supports that the narrator is critical of Scrooge is:
"The cold within him froze-his old features nipped his pointed nose shrivelled his cheek stiffened his gait; made his eyes red, his thin lips blue, and spoke out shrewdly in his grating voice." (Paragraph 6)
This description emphasizes Scrooge's physical and emotional coldness, highlighting the negative traits of his character, which reflects the narrator's critical perspective on him.