The reason Shauna is so bitter towards Lottie in the adult timeline is because Lottie was passively the reason she turned into the monster she was in the wilderness.

Lottie was the one who let Shauna tap into her anger and madness by allowing Shauna to beat her up. Lottie sides with Shauna during the coach Ben vote, everyone switches and Shauna starts feeling more powerful. Lottie makes her the new leader. Lottie killed the hiker guy, Shauna makes sure she doesn’t face any consequences. Lottie says she wants to stay, Shauna speaks up, then forces them all to stay. Lottie says they should hunt, Shauna goes into a frenzy, kills Mari and becomes antler queen. Shauna counts to Mari’s death and when she stops? Lottie picks up where she left off.

Lottie reminds her of how crazy she went in the wilderness and Shauna hates her for that.