I don't think that anybody would "like" the idea of having Fredo killed. That was the idea, I think. It was something so despicable, so ironic that Michael does everything as the Godfather for the good of his family, and he ends up killing his own family in the process. Yhst's the tragic beauty of it.

Killing Fredo was Coppola's idea? I'm not surprised. As a filmmaker, he will always be a greater fan of irony than Puzo, who was always more fond of succinct storytelling and was not too prone to literary devices.