Symbolism, Imagery, Allegory
One of the ways Finnigan exacts his revenge is by setting fire to people's stuff. Whether it's a house, barn, car, or outhouse, Finnigan loves to burn, and his fires quickly come to consume the town because—understandably—everyone wants to know who is ruining everything. It's not long before Constable McIllwraith and Gabriel's dad get in a turf war over solving the crime, which only leads to more trouble, causing rifts as townsfolk take sides. So on the surface, the fires pull apart the town and initiate fighting amongst its residents.
On a deeper level, though, the fires are a form of communication between Finnigan and Gabriel. Gabriel tells us that when "a crop was torched and the bell tower destroyed and the antique sign which welcomed tourists was reduced to a pile of cinders I understood this was Finnigan's sullen, wantonly violent, message to me" (9.44). The fires may destroy other people's stuff, but they're a direct message from Finnigan to Gabriel, letting him know which slight has been corrected or, later, that he's coming for him.
Whereas the animal imagery expresses how wild and uncontrollable Finnigan is, the fires are a reminder of how wild and uncontrollable Finnigan and Gabriel's relationship is. It is built upon a pact of destruction and, because of this—and because of Finnigan's wild nature—things will always be destroyed so long as the pact remains. Fires ravage whatever they can, and so does this friendship.