As painful as Fredo's death is, I think by the nature of Michael's business Fredo had to die. Even banishing him couldn't guarantee that he wouldn't take the easy way out again and give Michael up to authorities or something. Fredo knew a ton.

Fredo's death was so well done by Puzo and Coppola that it makes me angry to this day when Fredo is embracing Michael while sitting in his chair, like an innocent child, while knowing what his fate is. In the documentary "The Kid Stays in the Picture", Robert Evans says that the family situations in The Godfather were emphasized by design to make the film timeless, so that people like us would be talking about it to this day, nearly a half century later. This must be one of the scenes he was talking about.

The underworld knew of Roth's treachery to Michael, so, again, by the nature of the business, Michael had to make an example out of him.

Really don't have an answer regarding Pentangeli. His suicide was so bizarre. It's as if Michael knew that Frank was so old school Sicilian that he could guilt trip him into doing it for putting the family through all of the trouble he did. I love Pentangeli's line about taking everyone out, "while we've still got the mussscle!!!".