SB: I agree, I think a humble and repentant Fredo could've gotten away; but the showing of his emotional baggage meant that -to be left to live- he would be a constant danger in the hands on Michael's enemies.

Apple: your choice of words is impeccable, BETRAY is the word Michael used when making confession to Cardinal Lamberto "I betrayed my wife, I betrayed myself"...

Killing "your father's son" is betraying not only yourself, but the trust your father bestowed upon you when he gave you control of the "family".

The mafia was called "The Family" partially because it demanded trust from its members similar to that of real family members - and from what the movie tells us in the cases of Paulie, Carlo, and Tessio, there is only one punishment to the betrayal of "family trust"... Morte!

Fredo had committed the cardinal sin of betraying both types of family... it is a dilemma that Michael had to make up his mind about, and I think his mind was made up before that scene with Tom, Al, and Rocco... and before speaking to Fredo; you can see it...same boathouse, when Connie came in, knelt before Michael in the boathouse, told him how she wanted to come back to take care of him and the kids, and pled for Fredo; the look on Michael's face was that of a man who was troubled with the thought, mind made up but Connie's intervention led to him deciding to give Fredo a chance to speak; the next scene was with Fredo and his outburst,...then there was no doubt about what he would do, but its bothering him.