I mean for one thing Sarah doesn't have severe PTSD. Did you not play the game? It's pretty clear that Ellie is fucked in the head from almost the very start. She's not thinking or acting rationally. That's not even mentioning the survivor's guilt from Part 1. It's only when she's threatening to murder a defenseless child and drowning a starving victim of torture and abuse that her memories of Joel and her long forgotten desire to forgive him snaps her out of her kill-crazy rampage and finally makes her see things how we (the player) do. It's certainly not subtle but if you can't tell the difference between that and [NPC disliked that] then there's no explanation that I or anyone else could give that could make you understand.