It's a matter of degrees with recovery, I think. It's one thing to be redeemed at the end of Return of the Jedi, at the end of his life when he won't have to actually deal with the stuff he did. It's another thing to be "redeemed" early enough in the process to avoid Mustafar - he's still committed murder and still had to come to terms with being so thoroughly manipulated by Palpatine (and the Council, to an extent), plus the myriad issues that caused him to think turning would be a viable solution in the first place.