That's a good point, scarfacetm. Here's what I think: Sicilian Babe (in a past post on another topic) had rightly pointed out that Fredo also had a lot of unexpressed anger toward Vito, which he then directed toward Michael, who is now acting the role of "surrogate Vito."

It was Vito who never sat down with Fredo to explain why he was passing the torch to Michael, nor did he ever ask Fredo how he felt about it. It was Vito who assumed that Fredo was too simple to be bothered by this issue. Most significantly, it was Vito who didn't take the time to know his own son. Vito didn't realize that Fredo had pride, that Fredo had ambitions, that Fredo knew what people were whispering behind his back.

When Fredo exclaims that he was "stepped over," I think a lot of the sentiment in that regard is meant for Vito, as it was more his doing than Michael's that Fredo's entitlement was taken away from him without a word of explanation. If it weren't for Vito's underestimation of him, Fredo would at that moment be don, taking care of his kid brother, which to Fredo's mind is the natural order and the way it should be.

Michael's offense in the boat house is in not seeing any of this. (Fredo's question, "Did you ever once think about that?" comes to mind.) And this lack of empathy and consideration is, in my opinion, what makes Fredo's resentments -- toward Michael and Vito -- really spill out. I don't believe that Fredo wanted Michael out of the way so he could take over as don because being don on his own didn't carry the same meaning for him as being don with Michael under him. At this point, I think his ambitions were to be treated more like a co-don to Michael and for Michael to actually see him and consider him in the Family equation.