I think a crucial aspect of Mari and Shauna's hatred of one another that people often forget when they discuss their dynamic in season 3 is that Shauna's treatment of Mari was not unprompted--at least not to her.
Mari had been antagonizing, or at the very least aggravating Shauna for months, from Jackie's cremation to the baby's birth. The "wilderness, I hope Shauna doesn't die" comment, as well, comes off as incredibly insensitive when you remember that a) Shauna was extremely terrified in that moment and in need of comfort and b) Shauna was a staunch non-believer in the wilderness. From the viewer's perspective, and from the perspective of the other girls, Shauna's treatment towards Mari is totally uncalled for; but to Shauna, who holds deep grudges towards those who she perceives as having wronged her in that time (see Coach Ben), it's completely appropriate.
