In "The Black Cat", why do the narrator and his wife go down in the cellar?

The narrator does not give details of why he and his wife
went into the cellar. He simply says-

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 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.

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 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-

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 Goaded  by the interference, into a rage more
than demoniacal,  I withdrew my arm from her grasp and buried the axe in her
brain.