Giancana wouldn't kill Cerone as long as Accardo was around as Cerone was JB's protege which made him somewhat untouchable. If Accardo were not around Giancana may have had Cerone killed but I think their "feud" has been slightly exaggerated. There may have been some bad blood on a personal level but most of their animosity probably originated on a business level. Giancana knew that Cerone was positioning himself to potentially take over the Outfit one day which probably created some level of animosity between them. Once Giancana left and Cerone came back from jail, the latter's power increased tenfold. When Giancana returned, Cerone, and Aiuppa for that matter, were fearful that Giancana would try and regain his power (which he didn't show any inclination to be doing but Aiuppa/Cerone were taking no chances) so they had him offed. Certainly, this offered Cerone some level of satisfaction seeing as how he probably viewed Giancana as stonewalling his earlier efforts to gain power within the Outfit.