Like I say, I think it was a combination of Fredo being the black sheep due to his simplicity AND the after shock of being with the Don when he was shot. You have to admit, his being constantly left out from that point on, went a long way in his relationship souring with Michael.

Which begs many other questions. Had Sonny lived, would he have been as impersonal with Fredo as Michael was ?

Would Sonny have included Fredo more in the family business ?

Would Fredo have betrayed Sonny as he did Michael ?

And the big one, would Sonny have ordered the death of Fredo ?