Rue Morgue, Dupin can't deduce?

So my teacher asked us what can't Dupin deduce about the murder, then later he related it to Poe's other work, "Man of the Crowd," saying that if one cannot read or categorize a person, then that person is a criminal, and is that a fair assumption. However, my thoughts were, well first that I could not figure out what Dupin could not deduce ( it may be the motive, but personally that answer seems childlike). Second that when Poe explains that man the narrator could not categorize in the "Man of the Crowd" as a genius of a deep crime that the crime was that of not fitting into the anonymous society, and not a criminal. I can see that the man is a criminal, or assumed to be, because of the diamond and dagger, but I saw the story more relating to a warning of London in the 1840s and the growing issue of an anonymous society.