It wasn't about overthrowing Massino, when Rastelli died, the intention was initially to make Catalano the boss, but they couldn't because he was already a made guy in Sicily, apparently you couldn't be a made guy from Sicily and become a boss in NY during that time, as apparently people believed you couldn't pledge allegiance to two sides. So Bonventre couldn't be a boss anyway according to that logic. Massino killed him because he felt that Bonventre was too absorbed in himself, with the fancy watches, expensive clothes and sunglasses and cars. And mostly because when Massino was on the run after becoming boss, he felt insulted by Bonventre because neither he or the other Sicilians sent Massino any money while the rest of the non-Sicilian faction Bonnanno's, did.