By the end of 2 , Michael has
-alienated and humiliated his adoptive brother
-slapped his wife and rejected her love
-ordered (or at least tolerated) the death and mutilation of an "innocent" hooker
-threatened the innocent brother of a recalcitrant lieutenant
-ordered the suicide of a broken old man
-and finally ordered and watched the death of his hapless and traitorous(?) older brother.

It's not just about arguing for Fredo's life; it's also about arguing for Michael's soul. All of those actions Michael took made sense taken by themselves but added up they condemn the character to hell , primarily via pride. His pride in his intelligence made him unable to relate emotionally to what was going on.

Giving an enemy a pass is of course unusual for any oc figure, real or fictional but IRL Magliocco got one despite plotting to murder Lucchese, Gambino and Maggadino.

And since I'm not 100% convinced that Fredo knew it was a hit I am somewhat open to him getting a pass because he is a brother. If I thought for sure that he knew it was a hit and was in on it then I'd say what happened was meant to be.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.