I am absolutely convinced that Michael was in the dark about Fredo's complicity until they attended the Havana show. And I agree that fratricide was an extreme; exile was appropriate, but not murder. Also, consider the insidious way that Michael lured Frdo into a sense of forgiveness. That was really, really cold.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."