Spoilers
I really love this game and if you all think about it Ellie had already avenged Joel and yes she spared Abby but that is a smart move because she would had started another revenge cycle what if lev grow up more experience in combat to kill Ellie just like how Abby did Joel and not only that Abby has to think about how Ellie could killed her for the rest of her life i'd say thats the best revenge and instead of wanting the obvious to happen understand the game 1st and what is also a perfect twist of the game if Ellie didn't come for Abby for “revenge” Abby would had died tied up by the beach