A bedroom is not a structure, the building (house) is the structure a bedroom is a part of the structure. From http://www.legal-explanations.com/definitions/structure.htm A structure is defined as the physical development made in a land with a purpose to provide accommodation . For a construction or development to be a structure it must have an enclosure and a roof with provisions for entering within it. A structure is the first stage of construction of any building.
From http://legal-dictionary.thefreedictionary.com/structure anything built on land from a shed to a highrise.
Um, a bedroom has an enclosure, a roof and a provision for entering it. It meets that definition of structure. But that's beside the point because that's not the definition of structure. See Merriam Webster for the definition. That particular internet site isn't authoritative at all.
But granting you that for purposes of discussion, even it if is "part of a structure", isn't firing into the bedroom firing into a structure (that the bedroom is part of)? The law doesn't say you have to be outside the structure when firing.
From what I've seen so far and from reading Arkansas law, it's pretty cut-and-dried that this guy had no right to use deadly force.