From the text:
Tom lifted up his eyes and beheld a great black man seated directly opposite him, on the stump of a tree. He was exceedingly surprised, having neither heard nor seen any one approach; and he was still more perplexed on observing, as well as the gathering gloom would permit, that the stranger was neither negro nor Indian. It is true he was dressed in a rude Indian garb, and had a red belt or sash swathed round his body; but his face was neither black nor copper-color, but swarthy and dingy, and begrimed with soot, as if he had been accustomed to toil among fires and forges. He had a shock of coarse black hair, that stood out from his head in all directions, and bore an axe on his shoulder.
He scowled for a moment at Tom with a pair of great red eyes.
The description of Old Scratch infers he's a demon.... particularly the "great red eyes". The fact that he is "begrimed with soot" alludes to the fires of Hell. Would he frighten me? Defintely.