I think the ending is well done and is so divisive, that's why I love it. The opposite of TLOU2. TLOU2.. leaves you with no lessons, nothing to think about, no moral dilemmas, really. It's just "Revenge is bad!" I guess if TLOU2 does have a question it's "How far is too far for revenge?" But the thing is, TLOU1 already asked that question when Joel, you know, massacred an entire fucking hospital to save Ellie and kill those who tried to harm her. Including a doctor and Marlene.
As for TLOU1's ending, you look at the way Ellie is acting in the game and show and people think it's just the David ordeal. Of course, that is part of it. It was traumatic and will always be there underneath the surface to some degree. But there's two lines(one from the show, one from the game) that heavily imply what is on her mind most is survivor's guilt. In the show, Joel says to Ellie "You've been unusually quiet today." They had been traveling for weeks after the David incident and on that day she suddenly is quiet. What is it about that day that made her distant and withdrawn? To answer that we need to head to the game and a line they removed from the show, which I think is an important line and I'm not sure why they removed it. She tells Joel at the very end "I'm still waiting for my turn."
She blames herself for everyone dying. After all, they wouldn't have died in that same way if she wasn't around(in her mind). Her life and her being immune is the cause. I think in the final chapter, Ellie knows they're right around the corner from the hospital and it's starting to dawn on her: what if they can't create a cure? What if it doesn't work out? Suddenly, it won't just be Riley, Tess, Henry, Sam, etc. It'll be millions of strangers she'll blame herself for. And we see this in TLOU2 when she is really broken up by that choice being taken away from her. She blamed herself in TLOU1. In TLOU2 she blames Joel because the cure was finally something she could control of her own destiny and Joel robbed her of that.