When examining Shauna and Nat’s dynamic in Season 3, it really is so important to recognize the influence of Shauna’s nightmare/vision after the birth of her baby. In that dreamscape, which is entirely constructed by Shauna’s own psyche and therefore is very revealing of her own subconscious feelings, Natalie is the one who checks on her, the only person Shauna confides in, and the sole recipient of her vulnerability. When Shauna finally gets her baby to breastfeed, she and Nat share a heartfelt moment together, and Shauna tells Nat to keep the moment private, illustrating how Shauna views Nat as a trustworthy person.
Nat appears as understanding and empathetic in the dream, and it's notable that Nat is the only one with whom Shauna seems to let her guard down in this post-labor overprotectiveness of her new baby. Because this sequence is not reality but a projection, Natalie’s role in it shows how Shauna subconsciously views Nat as a safe presence and recognizes her as someone inherently good.
Then, when Shauna walks out into the living room to see the whole group eating her baby, Nat is the first person she focuses on. We get a close-up of Nat’s face, which appears remorseful and ashamed, while many of the others’ faces appear unapologetically hungry and frenzied.
Shauna sees Nat as the most remorseful in the horrific act of cannibalizing her baby, and she feels most betrayed by her due to the trust she placed in her. Shana knows Nat has the strongest moral compass and always tries to do what’s right. Shauna always trusted her because of this, and that’s why seeing her in this role feels like a deeper betrayal than anyone else. She recognizes that Nat is sorry, she's ashamed, she's wracked with guilt, but that didn't stop her from participating. This is the beginning of a pattern in which Shauna resents Natalie for presenting herself as a "saint" while being complicit in the group's sins.
After this, Shauna spirals into grief-fueled paranoia. She doesn't trust any of them anymore, as she can't erase the visceral image of them all consuming her baby, can't separate it from reality, and Nat becomes the emotional epicenter of that betrayal.
These two snippets from the 2x07 script (the first taking place as Shauna is burying her baby’s body outside, and the second right before Shauna beats up Lottie) are very revealing:
This is where grief and betrayal turn into suspicion, and suspicion turns into resentment.
From Season 3 onward, Shauna repeatedly tries to strip Nat of her false sainthood. She goads her into violence, pressures her to be the one to shoot Coach Ben, and forces her into the butcher role. She's trying to make Nat confront and publicly inhabit the same moral ugliness Shauna feels condemned to live with. Shauna wants proof that Nat is no better than the rest of them; she's spiraling into her paranoia and how deeply and irrevocably her trust was broken by Nat.
This is also part of the reason why she becomes enraged when she discovers that Nat has been lying to them all about knowing Coach Ben is alive. To her, this is just evidence of Nat's betrayal and her choosing her own needs over others while maintaining the image of sainthood.
Of course, there are so many layers to Shauna’s resentment of Nat beyond this (her lifelong feeling of invisibility/lack of recognition being reawakened by Nat being chosen as the new leader instead of her certainly being one of them), but Shauna’s post-labor nightmare and the paranoia that follows remain central to her arc and her dynamics with the others. That psychological break reshapes how she interprets trust, betrayal, and moral intention, and it bleeds into every relationship she has from that point on. I think it's equally fascinating and tragic that Natalie becomes the primary site where Shauna’s grief, paranoia, and resentment converge.