The Cruelty Within
The Muggles often say that people have different ways of coping, dealing with problems in their lives. For abnormally stupid, useless pieces of filth, they did have one thing right. If there was anything she could relate to Muggles about, it was coping.
She didn't break Mudbloods, Muggles, and half-bloods because of her beliefs. Do not misunderstand, she despised the despicable little creatures, but her motivation did not arise from disgust. Of course, she was loyal to her Lord and his vision, but that was not why she became a monster.
She grew up in a harsh, cold pure-blood world. Her parental figures were always aloof and reprimanding, impossible to please. She lost both her sister and cousin, simply because they made choices different from hers. Even her marriage was arranged, to Rodolphus, who she normally wouldn't touch with a ten-foot pole.
She never got to control her own life, make her own choices, her own mistakes. That was what she craved about the torture, the killing. She craved the power, the sense of control of holding someone else's life in her hands. And maybe, she understood why everyone was trying to control her. She understood why they were so desperate to ruin her, because they wanted to get a simple taste of that power, that satisfaction of holding someone else's fate in your hands, of crushing it until only shattered pieces remained.
People seemed to love hearing her pain, her screams. So she swore to make someone else scream in pain, in agony, for every time it had happened to her.
Maybe, just maybe, Bella wasn't as much of a monster as we think she was. Perhaps she was just as human as the rest of us, with a different way of coping.