Many films (and some books) would be exceptionally boring if one of the characters had made a timely phone call to the police. The Godfather manages to make it plausible that the police aren't called. For one thing, the police may be controlled by another mafia family!
So as I was reading the book, I began to see it as a story of what happens when groups set up in parallel with the state. The mafia is really an alternative government, a law unto themselves. The Godfather is an anarchist.
One may ask, what's wrong with that? Why can't they go their own way? The problems come when they interact with the rest of society. They are parasites on society. They make money from protection rackets, not by making their own contribution to the economy. If anyone stands between them and their revenue stream, they use violence to get their way, whereas legitimate business has to compete on a level playing field.