To me, the question is something of a false dichotomy. One of the fundamentally fascinating aspects of the entire Godfather saga is its moral complexity, how it juxtaposes the demands of environment and circumstance vs. personal character, and how it challenges the reader and/or viewer to put himself in the place of any of the characters, then answer truthfully: "How would I have behaved had it been me in that character's place?"

One of the most illustrative examples is the case of young Vito, growing up as he did in the Mafia-infested culture of late 19th Century Sicily, having experienced the trauma of the murder of his entire family at the hands of the local don, only to be whisked away across the ocean to a new land where he would quickly need to fend for himself.

Even as an honest young man in New York, his fate and his fortune were still being constrained and co-opted by the Black Hand. He finally began to thrive only once he took matters into his own hands, and began to play rough himself.

Vito's entire life up until that point had been one in which might makes right, only the predatory survive, and whatever you want in life, you must be prepared to take it away from whoever is keeping it from you. He never knew any other kind of life. Is it really any wonder that he became the person he ultimately became?