The narrator does not give details of why he and his wife went into the cellar. He simply says-
One day she accompanied me, upon some household errand, into the cellar of the old building which our poverty compelled us to inhabit.
It is evident that the wife trusted her husband, despite his unreasonable anger and strange behaviours.
The moodiness of my usual temper increased to hatred of all things and of all mankind;
He is humbled by her devotion and humanity, to him and to the cat he perceives as evil-
my uncomplaining wife, alas! was the most usual and the most patient of sufferers.
It is her humanity and goodness in trying to prevent her husband from taking his axe to the cat he is tormented by night and day. His wife intervenes as he raises the axe to the cat, and she pays most cruelly for her action-
Goaded by the interference, into a rage more than demoniacal, I withdrew my arm from her grasp and buried the axe in her brain.