I guess it makes sense. After the murder of the 3 capos, Rastelli was still the sitting boss (in jail) while Massino and Napolitano equally ran things for him on the street? I could definitely see Massino wanting take Napolitano out for sole control, and the Brasco thing was probably the best excuse to save face in the eyes of the rest of the family and other families. From what I read about Tony Mirra the guy seemed like one massive liability and Brasco could have been used an excuse to take him out as well. Disgusting that his own cousin had to do him in though...