The hatred that Ellie feels towards Abby is an externalized reflection of the hatred Ellie harbors for herself. Why? Well Joel deeply hurt both Abby and Ellie in different ways and they both punished him for it. Ellie hates Abby for torturing Joel to death, especially because she didn't get a chance to restore her relationship with Joel. This is a mirror because Ellie also tortured Joel, she did it for years but in a different way, and I'd say in a way that was more painful for Joel, and that was by disconnecting from him. She hates herself for torturing him like that and not restoring their relationship when there was still time. That's the real pain she has been trying to avoid facing during this entire revenge quest, she's like a drug addict using revenge to avoid dealing with those painful feelings of regret. This is the realization she has while she is drowning Abby when she has the flashback to the scene on the porch with Joel and chooses to spare Abby. She's not just forgiving Abby, she's really most importantly forgiving herself and finally coming to terms with what happened between her and Joel, I think her leaving his guitar to rest in the house and walking away is supposed to symbolically represent that she has forgiven Joel, made peace with what happened between them and is finally ready to move on and start a new life. I hated the end at first but after much deliberation I think it's actually really poetic. People saying the game is just about "revenge is bad" I think have really missed the point.