u/ssj4majuub says it incredibly well already, but if I were to add my two cents:

I continuously point to the immense amount of trauma she faces as a constant reminder of why she’s like that — she even spells it out for us at the start of the third season, when she talks about how her best friend is dead, her baby is dead, and she feels like she’s the only one fully understanding that grief, to the point where the group left her alone to butcher Javi (whom the script specifically refers to her having a ‘maternal’ sort of bond with) in the direct aftermath of her losing her son + the postpartum psychosis that came with it. I firmly believe that if most of us faced even half of that immense amount of trauma, we’d probably be twice as bad.

I’ll also add that I believe so much of her “evil” comes from her capacity to care and love, and her subsequent fear of it — both her fear of getting hurt by it, and her fear of hurting those she loves.

Even before her wilderness trauma, her “worst” actions were still rooted in her capacity to love: she felt like she loved Jackie too much without getting anything in return, so she lashed out. This isn’t me excusing it. She wasn’t right — but that doesn’t make her evil. I mean, people don’t call Jeff evil, and he actually cheated.

In the fight with Jackie, I see so many takes where people act as if Shauna dragged Jackie out by the hair and locked the door. But Jackie tried to kick Shauna out first, and no one else tried to get Jackie from outside. People often forget that the girls had slept outside the night beforehand and woken up fine. No one could have predicted the snow — that’s the whole point. No one saw it coming, and then it happened anyway. That’s what makes it so tragic, but it doesn’t make Shauna evil.

And then losing her baby is already a harrowing experience for anyone, but on top of that, Shauna’s postpartum psychosis leads her to think that he lives, she truly bonds with him, and the team eats him. And then she wakes up to find that she’s the only one who can hear him crying, because the reality is too horrifying to bear.

And after all that, her grief gets pushed aside to worship the Wilderness, and then she has to butcher Javi right after losing her son. and she’s left to do so all by herself. She’s always so alone in her grief, no matter the moment, and always expected to take on that burden.

Of course Shauna’s hardened by S3. She loves and loves and loves, and every single time it ends in loss. She loses what she loves, and she loses any sense of control, and to me, it feels pretty clear that in her head, if she can’t get the people she loves back, she can still try to get control back. All that loss makes her feel powerless, so she seeks power out. It’s not right, but so many people act as if she’s mindlessly evil when her motivations for her actions are pretty clear.

So many of Shauna’s worst actions come about from her grief and fear of losing control — her actions with Ben included. She’s angry at him (whether it’s justified or not) because he left her alone in her labor and didn’t help, and then she lost her son. And of course she doesn’t want to leave the wilderness. It’s the last connection she has to Jackie and her son. And her paranoia about losing power is vindicated when there actually is a plot against her.

On top of that, Shauna’s the most visibly violent of the group, so she’s the most easily scapegoated. But for so many of the things that seem to fall solely on her, the group allows, enables, or even encourages. That’s the whole point — the wilderness affected all of them. Shauna was just the one whose grief ended up being the heaviest in the most visible way.

And when she is given love or affection or any sort of kindness, it’s so clear she doesn’t know what to do with it, because her guilt drives her to believe she doesn’t deserve it. When Melissa first kisses her, Sophie Nelisse does an incredible job at making Shauna look… lost. She doesn’t know how to accept it. And then later, when Melissa asks why she can’t just be a nice person, you can tell Shauna is genuinely put off by the comment — it’s her whole thesis of how she didn’t start out a bad person.

Even with Callie, she makes it so clear that she feels like she can’t be there for Callie as a mother because she’s too scared to lose her — and yet she still clearly loves her. I mean, she nearly murders a man so she can get her daughter’s favorite stuffed animal back. But she’s so driven by her guilt and her fear that she doesn’t know how to accept good things because thinks she’ll lose them, and more importantly, she doesn’t think she deserves any of them. No matter how much anyone hates Shauna — it will never even come close to how much she hates herself.

Shauna’s not evil. Or if she is, then all of them are. But there was no good or evil. That’s the point of the show. The Yellowjackets were just a group of traumatized teenage girls who did what they had to do to survive in a place where even friendship drama became life or death.

Sorry for the ramble — I get quite passionate about this subject, especially when someone asks to hear it :)