The following contains spoilers for One Piece "Whole Cake Island" Arc, available in English from Viz Media.
Being an Emperor of the Sea, Big Mom is one of the greatest adversaries the Straw Hat Pirates face in One Piece. She's a menacing, sweet-toothed giant who lets absolutely nobody get in the way of satisfying her cravings. When a good number of the Straw Hats infiltrated her territory and sabotaged her tea party, she naturally had no plans of letting them leave the island alive. Given her despicable, spoiled behavior, Big Mom has been a target of many since childhood. Even so, one character has shown her what affection is like, and that's her foster parent Mother Carmel.
Mother Carmel was a missionary nun known as "Holy Mother." She took in despicable, unlovable orphans and gave them shelter in her orphanage. One of those orphans was the young Linlin. The nun took her in while she was still a 5-year-old brat and catered to her wants and needs. Arguably, that was the greatest year of Big Mom's life. However, that didn't last for too long. During Big Mom's sixth birthday, the nun and all the other orphans suddenly vanished. And Big Mom herself is clueless as to where they've all gone.
Big Mom Ate Mother Carmel and Her Fellow Orphans
When Big Mom was explaining how her foster family suddenly vanished, the manga made it seem that Mother Carmel and the children simply left the young Linlin. After all, she was crying in ecstasy at the time. She was so delighted by the semla croquembouche the other orphans prepared for her that she couldn't see through her cascading tears. She merely kept eating until she was satisfied, and when she could finally open her eyes once more, everybody else was gone. While the manga didn't explicitly explain what happened to the nun and the orphans, there were numerous hints pointing to their gruesome demise.
Instead of leaving the monstrous 6-year-old Linlin, the reality is that the young Big Mom cannibalized Mother Carmel and her foster siblings. And that occurrence was the first of many other binge-eating sessions. When Linlin opened her eyes, what greeted her wasn't peaceful abandonment. The place was messy, with debris littered on the ground. Linlin herself admitted to eating pieces of the table in her delight. However, the table wasn't the only supposedly inedible object she consumed. Among the debris was tattered clothing with clear bite marks on them. In addition to that, it was after that incident that she ultimately acquired her Soul-Soul Fruit powers — the very same Devil Fruit power that Mother Carmel possessed.
Big Mom is Who She is Because of Mother Carmel
Mother Carmel might seem like a big-hearted philanthropist, but she was far from being a saint. While it was true that the old nun took in and fostered numerous troublemakers, she only did so for her own benefit. She was only known as the Holy Mother on the surface, but underground she was an infamous child trafficker. She sold the children she was raising to various forces, including the World Government. Given that Linlin was her biggest catch, the young girl was also supposed to be her last gig. Unfortunately for her, she was eaten before she reaped the product of her hard work.
But despite her true motive, it doesn't change the fact that Mother Carmel is the closest person to Linlin, as well as the former Emperor's role model. Mother Carmel died before Linlin learned about her true identity. Even at present, Big Mom sees her as nothing more than a loving mother figure, so much so that seeing Mother Carmel's picture getting destroyed sends her into shock. Big Mom is also trying to create a world where everybody, regardless of race, can look at each other on equal grounds, since that's the core value Mother Carmel instilled in her. While that doesn't absolve her from her sins, it is arguable whether the nun received what she deserved. After all, One Piece is ruled by crime and violence.