I would not have Ellie kill Abby. I’d keep the ending the same but have Abby be the one with the upper hand at the end of the fight, then she decides not to kill Ellie. It makes more sense to me that Abby would see that killing Ellie will just perpetuate the murder cycle, as Abby did when she killed Joel. That’s something Ellie can’t admit to herself because she lets her anger guide her actions, even when it hurts her friends and loved ones. By letting Ellie go you are robbing Ellie of her vengeance, making her sacrifices pointless, which would hopefully show her that her violent ways only lead to violent ends for her and everyone around her.
I feel like that would be a better ending because Abby seemed more like the hero of the story than Ellie did. Ellie is definitely the villain to me because at a certain point in the game I stopped sympathizing with her. I think that is why Ellie having a change of heart at the end felt so off to me. We just helped her kill hundreds of people without shedding a tear. That person would not stop when they finally had their chance for revenge, especially with what it cost them to get there.
Most of us just like the day off from work, or the holiday pay if you have to work. and that’s where it ends.