Fredo signed his own death warrant with the "It's not the way I wanted it!!" rant at Michael. First of all, that was of course a direct challege to Michael, and just as important it was a direct slap at Vito's judgment.

Once Fredo blurted that out, Michael knew he was never going to be satisfied with his role in the family and would always be ripe for being manipulated by an enemy of the family.

I think up to that point if Fredo had come clean, Michael would have let him off - cast him out of the family business for sure, but at least let him live.

But once he delivers that rant, he's done. Michael gives the message to Al Neri right after that. He's dead meat.

Back to the original topic, if Sonny were still alive, Fredo wouldn't have been "stepped over" so it would have been a moot point. But Sonny of course has that infamous insane temper. If Fredo had betrayed him again and again as he did with Michael, I think eventually he might have done it. But it never woulda come to that.