Gotti IMO was taking out whoever he needed to in order to become boss. Gotti knew it was kill or be killed and he struck first. Keep in mind that Castellano was Sicilian and HATED the fact that Gotti was Neapolitan. As a first generation Italian, my famil is Calabrese and it amazes me how men in certain regions of Italy look at other men from a different region. Neapolitans are known for being "serpents" and the Sicilians never trusted the Neapolitans. Guys from Castellano and Gambino's generation took that stuff very seriously.