How we cite our quotes: All quotations are from The Godfather.
Quote #4
CLEMENZA: Don Corleone, you once said the day would come when me and Tessio could form our own families. Until today I would never think of such a thing but now I must ask your permission.
Clemenza asks him for permission since the Corleones seem to be on the decline. It shows that the gangsters' bonds aren't really as unbreakable as unconditional family bonds. They're much more business-oriented and transactional.
Quote #5
FREDO: Mike! You do not come to Las Vegas and talk to a man like Moe Greene like that!
MICHAEL: Fredo, you're my older brother, and I love you. But don't ever take sides with anyone against the Family again. Ever.
Fredo makes the mistake of going against the family. Even if Moe deserves more respect and isn't such a bad dude, family takes priority: it's above right and wrong, and all conventions.
Quote #6
MICHAEL: […] Only don't tell me that you're innocent. Because it insults my intelligence and it makes me very angry. Now, who approached you first? Barzini or Tattaglia?
CARLO: It was Barzini.
MICHAEL: Good. There's a car outside that will take you to the airport. I'll call your wife and tell her what flight you're on.
Of course, Michael has Carlo killed a moment later. He committed the ultimate sin: he went against the family, betraying his own brother-in-law. There could never be any forgiveness for that.