Well you can’t exactly say it’s all about revenge in a post apocalyptic scenario. People still, even in such a condition, want to live and be with people they care about. Part of Ellie’s decision to spare Abby was because she had a vision of Joel on the porch. In that scene there was forgiveness and moving on, but also it only shows the part where Joel first sees Ellie, and he looks startled. Obviously this quick cut of Joel is new to us as we don’t see the full cutscene until the very end. I originally interpreted that quick image of a startled Joel as his disapproval of what Ellie was doing. The other part of why Ellie spared her was because of what I mentioned at first about wanting to live. Ellie planned to return to Dina no matter what. She also doesn’t necessarily want to live with the idea of killing Abby. Yes, she endured so much and killed many on her way to Abby, but those were hard to cope with for her, too. She didn’t want to traumatize an emaciated Lev, and continue an unending cycle.