We're saying the same thing, then. I do not disagree that the fireflies believed. That is clear in part 1 as well. They are delusional. Nor do I disagree that Owen and Abby and them would want Joel dead.
I don't quite agree on the theme being belief keeping people going. Owen is grasping for any sort of meaning in his life. Manny does just fine without belief as does Nora. Ellie doesn't believe in anything, and Dina isn't a believer either. When belief does come up it is shown to justify doing horrible acts. The Fireflies murdering Ellie, the scars and their Messiah, the Wolves and their victory for the greater good at any cost. All of those systems are shown as horribly flawed.
Yeah, but everyone is flawed in this harsh, dog-eat-dog world.
Consider the history and timeline, Jackson may seem like an oasis of civilization but its not been around for long at all. We were there in the first game when they finally restored electric power, a mere 5 years in the past. Back then they were still on a highly defensive footing, fighting off marauders who threatened them
The point I'm getting at is if you scratch the surface there's blood and dead-bodies right underneath. Dina's pretty nonchalant about her backstory, but its still quite a history of violence and mayhem. You can assume that most people above a certain age will have similar tales to tell.
As to the matter of belief:
Ellie's long held belief that her immunity has to mean something is crushed when she discovers what Joel did. Which is why she lashes out at him for 2 long years, until on the cusp of them healing, Abby's vengeance takes that hope of reconciliation away.
Joel for his part puts up for it. because all he believes in is his role as Ellie's protector and father-figure. He goes to his death upholding this, so absolute is his conviction.
At this point Ellie starts out on her path of vengeance, but its less about justice for Joel than her managing her own guilt as to how she rejected him and never got chance to truly resolve their conflict. Getting revenge on Abby is important as it gives her a focus, but as Tommy and Dina and Jesse get pulled into the fray her resolve wavers, especially so after she finds herself doing things she finds hard to reconcile with her moral self-image,
Ellie is very conscious of the corrosiveness of her chosen course, but there's an element of self-punishment derived from her guilt over the state of her and Joel's relationship.
By contrast, following her father's death Abby commits absolutely to her path of vengeance. She shapes her body as a weapon, hardens her heart, sacrifices her relationship with Owen and literally lives to exact her retribution on the man who wronged her.
After killing Joel however she discovers that her nightmares don't go away. Completing her "mission" hasn't given her peace, even though she can finally move on in her life.
After being saved by Lev and Yara she finally finds a reason in saving them and escaping the Seattle war zone. The key shift here is that she finally recognizes her error in obsessing for years on revenge. This is spelled out when she finally confronts Ellie at the theatre, she bitterly tells Ellie that she
wasted the opportunity afforded by her being spared back in Jackson.
Wasted is a very specific thing to describe it as, implying disappointment more than anger over how things have turned out.
Anyway after the fight, and her heeding Lev's appeal not to kill Dina (who lets not forget Abby has never set eyes on before she gets jumped by her during their fight) she finally breaks her cycle of vengeance, liberating her to go back and try and be the person she was before her father's death. She and Lev go looking for Fireflies, an unrealistically optimistic errand...
Meanwhile, Ellie survives but her spirit is broken. She remains left behind not because her immunity means something to humanity, but because this beast-woman who killed Joel has spared her twice. This is the route of her PTSD, Abby has basically taken her power.
When Tommy turns up with info, she has to go and face her one last time, because she knows if she doesn't she's going to be forever haunted by her failure, her inability to protect herself and those she loves from harm.
This is absolutely crucial. Ellie has no choice, to say no is literally slow death to her soul.
So... (nearly there now lol) When she finally catches up with Abby, and discovers the monster she's been chasing to be a shadow of her former self she can't just kill her in cold blood. She needs to face her fear, as only by conquering that can she finally move on.
During the fight we get two flashes of Joel, one showing him in death, the other in life. These signify the choice she has to make: Killing Abby at this point isn't what matters at all. Beating her is enough, the real question is what happens next. Where is she supposed to go after here: death or life?
Ellie at the last, chooses life and saves herself.
She does this by accepting Joel's love, the thing she spitefully rejected to punish him for stealing her destiny, and finally forgiving him. The tears she sheds in the water are for him, not for herself.
The epilogue, far from being a nihilistic dirge, shows Ellie finally able to peacefully reconcile with her and Joel's past. She may be battered, missing fingers, but she is no longer
left behind.
She is
still here, and that in itself is a triumph.