I can't buy into the "Neri felt sorry or Fredo" theory at all. There is no evidence for it. In one scene Michael tells
Neri he doesn't want anything to happen to Fredo while their mother is alive. Neri nods. In another scene Fredo hugs Michael at their mother's funeral and Michael shoots Neri a look telling him to ignore this hugging nonsense. Neri nods slightly as if to say "I understand) and then casts his eyes down. Finally Neri brings the boat around and as soon as Anthony leaves, he obeys Fredo's instruction to go out fishing.
If there were a scene where Neri even asked Mike what he meant
by all this there might be an arguement that Neri disagreed with Mike. But I see no evidence.
I also cannot buy the idea that Fredo was just a stupid bumbling idiot. He betrayed the family and as long as he was alive he would remain angry and bitter. I believe he even lied to Mike in the boathouse about not knowing it was going to be a hit. Fredo was a clear and present danger to the family and he had to go just like any traitor.