How do you explain the narrator's behavior at the end of "The Yellow Wallpaper"?

The woman in "The Yellow Wallpaper" becomes obsessed with
the patterns on the wallpaper and begins to identify with them.  To specifically answer
your question, the narrator is trying to free the woman trapped in the pattern, by
tearing the wallpaper down.  Of course, this is symbolic of the narrator's own
entrapment. 

When she can't peel all of the wallpaper off,
she begins circling the room as if she is trapped inside the wall and following the
pattern of the wallpaper.  By the time her husband comes home and faints after he sees
her crawling against the wall, she has lost touch with reality and appears to have
become the woman in the yellow wallpaper. 

That, I hope,
explains the narrator's behavior at the end of the story.