Is the unpardonable sin which is inside the breast of Ethan Brand the devil?

The Unpardonable Sin, which Ethan Brand went on a quest to find, is not the Devil. Bertram, who is new to town and doesn't remember Ethan Brand from when he previously lived there, had heard tales about the strange man who used to work over the lime kiln.
People said that Ethan Brand conversed with the Devil within the flames of the kiln. When Bertram begs Ethan not to bring out the Devil, Ethan scoffs. He replies, "What need have I of the Devil? I have left him behind me, on my track." He claims that the Devil pursues "half-way sinners" like Bertram but ignores Ethan, who has the Unpardonable Sin in his own chest.
If the Unpardonable Sin in Ethan Brand's chest is not the Devil, then what is it? Ethan identifies it as:

the sin of an intellect that triumphed over the sense of brotherhood with man and reverence for God, and sacrificed everything to its own mighty claims!

It seems that Ethan Brand decided to elevate his own intellectual pursuits above moral concerns. He set himself to gain knowledge but did so at the expense of other people.
One example given is Esther, a young woman who ran away to join the circus. Ethan Brand had met her on his travels, and he made her "the subject of a psychological experiment, and wasted, absorbed, and perhaps annihilated her soul, in the process."
However, the story is ambiguous as to whether Ethan Brand actually found the Unpardonable Sin or whether he only thought he had. When the Jewish man shows Ethan the supposedly heavy sin in his magic picture box, readers aren't told what he saw, only that a youth who "peeped in almost at the same moment" saw nothing.
Next, Hawthorne describes a seemingly irrelevant scene of a dog chasing its tail but never catching it. Both events hint that Ethan didn't find the Unpardonable Sin because, in fact, "man's possible guilt" cannot extend "beyond the scope of Heaven's . . . infinite mercy." Hawthorne's point, subtly made, is that God is greater than the Devil, and God's mercy is always greater than man's guilt.