Why did the dog bite the man from Islington?

At first, it seems that the dog bit the man simply because he went mad. Nothing unusual there, we might think. Biting people is what mad dogs often do. Indeed, just about everyone in the poem thinks that this is the reason behind why the dog bites the unfortunate man from Islington.
The good folk of the parish would say that; they are rather biased toward the man. Everyone regards him as having a "kind and gentle heart." The very idea of a dog biting such a benevolent individual for any reason other than sheer madness is something they simply refuse to contemplate:

Around from all the neighbouring streetsThe wondering neighbours ran,And swore the dog had lost his wits,To bite so good a man.

The suspicion remains that the gentleman from Islington is not as pleasant a character as everyone seems to think. Perhaps the dog knows something about his true nature that no one else does. Our suspicions are confirmed by the last lines of the poem:

But soon a wonder came to light,That showed the rogues they lied:The man recovered of the bite,The dog it was that died.

The fine, upstanding people who judged the man to be good and the dog to be mad have now been exposed by the speaker as lying rogues. The fact that the dog died after having bitten the man tells us a lot about what kind of person he really was beneath his kindhearted exterior.
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