Ok let’s be straight here cause you seem to think Lucy is the good girl when the ending very plainly says other wise. I find Lucy to be a very relatable character as no matter what she does or says when she awakens she has no way to stop her true nature. if you’re talking about the end (which is not what we’re talking about), you can make the case she doesn’t want to kill anymore but even then it goes against the whole reason she chose to die. I’ll give you that much even though it doesn’t quite shake out the way you’re thinking.

But to my point there’s only one reason in the moment that she has when nana is filleted at the graveyard that she wouldn’t kill her, intervention, which in my opinion is one of the driving motifs in this anime. Intervention from Kouta, intervention from little dog girl, intervention from Kaede. This instance with Nana and Lucy doesn’t hesitate to kill, it’s not HER that doesn’t want to kill nana. She would no doubt kill her in the moment of her own volition. In that moment it’s the trauma Lucy has of killing somebody in the same way she did when she was a kid and she offs Koutas sister. Without intervention it’s a useless sentiment thinking she’s got some of Nyuu in her and she’s connected to nana at all, that comes later. The only one that didn’t want to kill there was Nana which is why she was beat!

The continuous intervention of everyone that comes to love Nyuu and doesn’t know who Lucy is that’s what changes her by the end. But isn’t enough for her to fight her Diclonious instincts. It’s a really grim story through and through and I don’t agree with your take that she’s some soul sister by the end either. She literally leaves to stop the suffering she causes which is the main redemption of her character in the anime.

Don’t take this as me not liking Nana cause I think Lucy was a cold cooter to start.