And truth be told, it actually was Vito, more than Michael, who caused a resentment to build up in Fredo. He chose Michael to be his successor. It's the way POP wanted it.
In fact Michael probably gave more freedom and responsibility to Fredo thatn both Sonny and Vito combined ever did!
I think we have touched on this before, but I think it was the way Michael treated Fredo that upset him, not the fact that Michael passed him over as the head of the family business. There's a really touching scene where Fredo tells his half dead father that "I'm gonna learn the casino business." Fredo is actually proud of himself even though the truth is Sonnny is just getting him out of the way.
If Michael had handled Fredo with more kindness, Fredo would not have betrayed him. Look how remorseful Fredo is in Havana when he says to Michael "Why didn't we spend time like this before?"
If they had, Michael would have had Fredo as a loyal ally and not as a resentful younger brother who felt rejected.
