... Michael himself clearly regretted killing Fredo later in life. In his confession to Lamberto, he explains the killing by saying "he injured me." Not, "he was a danger to me" or "he turned against the Family." ...
While it was a well done scene and nice to refer to ... it's almost silly to factor anything in GFIII into this discussion.
Much GFIII was simply a wink and nod or to placate loyal fans of GF and GFII.
While of course Michael would regret it 'later in life'...he may have also regretted it a week later. He may have regretted it the moment he hung his head at the sound of the gunshot. His remorse isn't the point. The point is he handled Fredo's betrayal the way it had to be handled. By eliminating the traitor.
It's clear that Michael "regretted" it by the end of Part 2. The entire flashback is from Michael's point of view, and it's GUT WRENCHING.
But that has nothing to do with any of this. As a Mafia boss, Michael did what he had to do. As a brother or as a friend or as a human being, well, that remains to be seen. But Michael heading up a crime family supersedes all those other relationships. Fredo had to go because, in the words of Tom Hagen (the novel), "Treachery can not be forgiven."
Period.
