Mormon reprises his information about the link between the murder of Cezoram and his son and the Gadiantons because he is emphasizing this aspect of the Gadiantons. It is an essential part of his definition of the Gadianton robbers that they acquire their political influence through murder, so he returns to the first instance as his base proof for his developing narration. This prepares us for the next murder that will involve Nephi in chapters 8-9.