I feel like an important aspect to understanding Shauna this season is to remember that she did let herself be broken and vulnerable and soft after Jackie died and even for a little bit after the baby died. And coming from that experience, she learned that she's not allowed to show that she's hurting. They treated her like she was crazy for what she was doing with Jackie in the wood shed (objectively, she wasn't acting in her right mind at all, but it's not like a bunch of high school girls were able to handle that with nuance!) and they pushed her to let go of the baby too. In Shauna's mind, she's tried to be soft and the group told her "no, we need you to suck it up... and now butcher this kid for us..." And then they all wonder why she acts heartless now.