Other than being a technical masterpiece, what sets this game apart from other revenge stories is the context and perspectives it forces you to see and at least understand, even if you don't per se agree with them.
In the end it's about two people with similar stories of revenge, and the darkness that fool's errand has brought upon them. It also begs the question as to how far is too far, and how much are you willing to lose to right the ones that have wronged you - and the finale brings clarity to this theme in an emotionally shattering fashion. You struggle to fathom how somebody could steep so low or be so consumed with hate that there is no line left for them to cross.
SPOILER Warning:
Abby, the young women (who isn't Transgender by the way) who murders Joel to avenge her father, the Fire Fly doctor ready to perform surgery on Ellie to develop a cure, end up feeling hollow and dissatisfied with her crusade of vengeance. She sidelined living a happy life with her boyfriend and ultimately losses him because she can't move on, choosing instead to rigorously train and built up her physique. When it's all said and done, her actions much like Joel's before her have consequences, and at the hands of Ellie and Tommy, loses all of her friends, former Fire Flies, who helped her embark on their journey to kill Joel and avenge the Fire Flies (and her slain father).
Ellie meanwhile, equally consumed by hatred, crosses lines you'd never expect her to cross. By the end of the game, after ruining her own relationships and losing almost everyone around her - she still can't let go. She's haunted by Joel's death and believes that finishing what she started is the only path to salvation.
By the end you see both characters at near rock bottom, with almost nothing left to live for. One has had enough, the other isn't done yet, not until on the brink of eternal darkness does she relent and realize what she's become. The story ends with emptiness, but emptiness earned through not knowing when too far is too far, and the repercussion brought forward by those actions.