I want to first point out that Clemenza was my favorite character from the trilogy and the thought of him turning on and betraying the Corleone Family is repulsive to me. I don't want to think it could ever happen BUT I do think it would have happened if Castellano appeared in Part II.

Part II was about change; how times had changed loyalties whether we liked it or not.

One other point, as I see it.... FFC left Clemenza's death vague on purpose. It was FFC's way of getting back on Castellano for not coming to his terms on appearing in Part II, just as FFC didn't include the photographs of Brando and Castellano on the crime charts in Part II's Senate hearing scene. FFC is a vengeful fuck.