Quin's account about the incident five years ago was incomplete, but it made Geale realize that what he feared about the event was likely true. There was still a few pieces of the puzzle that he hadn't quite put a finger on. One of them was the fire. Who could have set it?
The smirk on Quin's face when he explained that the fire wasn't there after they retrieved Marnthe made Geale wonder if he had something to do with it, but Eli was his friend. Most of the people involved were humans. Did Quin have it in him to inflict such carnage?
Then again, Quin was human: a being with a propensity for hatred, greed, and violence. Despite the kindness, warmth and humility Geale had experienced from the humans around him, it still didn't erase the fact that crimes happened every day - that humans constantly inflicted harm on fellow humans. Quin might actually have the capacity to burn the criminals and frame Eli.