That said, once Fredo's plotting or mistake , led to the almost deaths of Michael and Kay, I'm not sure that any mob boss could have forgiven that.
Vito was far better at handling people in the sense he got people to like him, but if said weaker brother stepped out of bounds, putting him in harms way, I don't think he would have ever considered fratricide. I tend to think he would have marginalized him without committing murder.
In film I don't recall Vito blaming Fredo for fumbling his pistol and failing to protect during attempt on his life. He knew Fredo's limitations and cut him slack, though we have to question the wisdom of using Fredo as a bodyguard.
Well only FFC knows for sure at this point. It would be nice if he spoke on it some day.
But there's a big difference between plotting and a mistake. The film remains ambiguous enough so that there are good arguments on either side. For even the most egregious mistakes I can't see Vito ever lifting a hand to kill his own blood. That's unthinkable for that character. I would bet that when he's shot he's calling out Fredo's name as much to warn him as a cry for help.
But as has been argued elsewhere and will be debated forever, we don't know how much Fredo knew or what he did in regards to the Roth situation. Certainly the boathouse scene reveals a man that still has a tremendous amount of anger at his brother and is evidently not smart enough to realize that his life hangs in the balance by the words he says. Both Vito and (to a lesser extent) Michael will accept mistakes and even incompetence. But disloyalty is a different thing.
Fredo says "you guys lied to me" and tells Michael "I didn't know it was gonna be a hit". Well what did he think it was gonna be? Did Ola/Roth really lie to him or did he lie to himself? Ola says "We need some
more help". Well what "help" did Fredo give them before? Who killed those men outside Fredo's home? Did they go there to report to Fredo or have Fredo hide them/help them get off the property?
We don't know.
All we know is that Vito never would have had to deal with this sort of thing-partly via luck but mostly because he didn't inspire resentment and jealously within his blood family. And if he did Fredo still had enough respect for his father not to raise those issues.
