Well, if you think about it, there is nobody else in better position to know the truth than Joe Massino. He may have told his underlings like Vitale, Lino and others (and perhaps Rastelli too) that Sonny Black was killed due to him introducing Pistone into the Bonanno Family, which would be a "justifiable" cause to kill him. But Massino may have had a more sisnister motive, not revealed until he gave testimony on the hit. He may have been jelous at Sonny Black´s power and growing closeness to Rastelli and feared a power grab. I read somewhere that sometimes when Massino and Sonny Black met and talked, the discussions often ended in shouting matches.