I can see where you're coming from and while playing the Ellie section and having no idea of the motives I just assumed Abby was some firefly grunt.
That first chapter of Abby's story hits though, it has impact. You realise that her motive is something you, having played Joel, had rationalised. You don't need to play an entire game to understand what she did it for, you already have. It's a great sequel because it reflects the actions of the original.
She did what she did because Joel in a way deserved it, this is something that even Joel understands, he surrenders to it. Actions have reactions. You as Ellie then go on to do the same, because violence cycles and continues. An eye for an eye until the world is blind.
I kinda disagree though. Joel might have done bad stuffs and understood bad actions have bad reactions.
ND might have want us to sympathize with Abby, but it didn't work. Remember, ou played as Abby, Ellie doesn't get to play as Abby.
For Ellie, she only knows that her father figure has been killed in front of her eyes, even as she knew the reasoning behind Abby's motive, especially also when she knew all the crazy things WLF and Scars did.
And many time in the game she asked whether she is backing down from the revenge, she doesn't. Only until a certain part where they decided that helping Dina will require a compromise, give up the revenge and head back, then Abby comes back and kill Jesse in an instant (doesn't even hesitate like Ellie), and also wounds Tommy. At this point, no sane characters would make peace. You don't feel the impact of what ND wants you to feel because there is no agency of choice, you have to do exactly as the plot dictates.
Even if they did want to make peace, they wouldn't travel 99 miles, and just turn back when the destination is 1 metre in front of you, because you remembered your father, whom you understood that if it were him, he would never had stopped at all (as mentioned that he would be halfway across seatlle day 1). "An eye for an eye makes the world goes blind" doesn't make any sense here, she would have stopped much earlier, and certainly not because of a memory of Joel. It would have made more sense if she failed.
ND made a very big build up on the relationships of the characters, only to have it done the 1 thing they will never do in the situation. In the end there's no payoff, no meaning, just plain "what??!"