IMO Fredo seemed to being showing genuine remorse in his "you're nothing to me now, Fredo" chat with Michael, despite the anger he showed with his "that's not the way I wanted it" comment.

Sure, he was angry, but he had to realize that that wasn't "the way Pop wanted it" and that Michael's taking over as Don was not Michael's doing.

Or, perhaps, he was too stupid to realize even that.

Also, I think MP and FFC painted a deliberate picture of everyone living happily in Tahoe subsequent to mama's death, in order to reinforce the idea that Michael had, indeed, become a heartless bastard.

So no, i don't think Fredo would have intentionally done anything in the future to betray Michael.

However, stupid as he was, there was always the possibility, and hence the danger, that he would inadvertently done something.

BTW, although Fredo was obviously not killed in the novel, at the end of the book, in the scene where Tom goes to New Hampshire to bring Kay back, he tells her, in speaking about treachery, that

Michael could have forgiven it, but people never forgive themselves and so they (Tessio and Carlo) would always be dangerous.

If you agree with that statement, then Michael was justified in killing Fredo. I do not, however, agree with it.

If you want to argue for revenge as a legitimate justification for Michael killing him, I can't argue against that. If Michael felt he had to have his revenge, even against his own brother, so be it.

But if all he had to worry about was an inadvertant betrayal, I would say that he should have let Fredo live.