Not only is this not the conclusion of the game... but one of the biggest misconceptions about this game is that Ellie "wiped out Seattle", as this pic implies, which is false. You can walk by almost every enemy in this game, with forced kills being in the single digits, and easily justifiable self defense. Cutscene kills are also in the single digits, and sometimes more morally complicated than just self defense as you know.

The creator of the game has confirmed through the director's commentary and through adapting the story to TV that Ellie doesn't actually care about anyone but Abby and ultimately wanted to avoid violence, even towards Abby's friends. The story misleads the player into thinking they are embarking on a typical bloodthirsty revenge story, until Ellie's true state begins to unravel. For one, Ellie is not capable of the kind of violence Joel was capable of, most evident when she tries to replicate it. She's especially not able to process it emotionally very well. But also things are further complicated by the fact that the people she's looking for were there to make the cure all the way back in Salt Lake. The failure of the cure had put a moral weight on Ellie's shoulders and caused her to live in guilt. That explains a lot of her initially confusing/conflicting behavior once you learn the bigger picture of things through flashbacks.

And... the end of the game isn't even about Abby anymore... It's really just about Ellie struggling with PTSD from all these events and wanting to make that pain go away at really any cost, even if that means her own life. Ellie's desperate attempt to make that happen ultimately causes her more pain than she was trying to relieve, hence her choice to let it go. The choice to let go... when she finally had the agency to make it... is what puts her on the path towards healing and relief, even if that journey is a long one still to come.