Its a fact that they started as a Bonanno crew, but in time they got more independance from New York, because of the attacks from the feds/gov. Hence, they became the "Sixth Family". Their family is build upon the ruins of the Bonanno crew. Thats why they are made Bonanno members, maybe a bit renegade.
I realize they started out as a Bonanno crew. The debate revolves around whether they stayed as such or whether they evolved into their own independent crime family. From what I can tell, they were quasi-independent - for lack of a better term. And that due to geography as much as anything else. You had guys who were nominally Bonannos but practically independent for the most part. Guys who weren't American LCN at all but tied to the groups in Italy. And so on. At the same time, I never really bought into the "Sixth Family" hype said about them over the years. Even before their decline began.
I agree, we could say that the real thing that broke the chain was the killing of George from Canada. That was the last thing, and the Rizzutos were looking for a reason to say "Fuck you New York". And thats what they did with the killing of Georgie.
