One thing can be stated with a lot of confidence: Madam Lockton is a mean woman. What is open to individual reader opinion is exactly why Madam Lockton is so mean. Personally, I think she is just plain evil. Isabel recounts her mother's support of that notion in chapter 43.
Everybody carried a little evil in them, Momma once told me. Madam Lockton had more than her share. The poison had eaten holes through her soul and made room for vermin to nest inside her.
Of course, maybe Madam Lockton is sadistic for another reason. She is an inherently selfish woman, and that selfishness does not do good things for her marriage. Often, Madam Lockton's selfish and harsh demeanor does little to endear her to Master Lockton, and angering him is never a good idea. He mistreats Madam Lockton as badly as she mistreats Isabel. Master Lockton beats her in chapter 17 until she has rings under her eyes.
It is possible to think that Madam Lockton is so mean to Isabel because Isabel is someone that she can exert her power over. Madam Lockton knows that she can't exert power over her husband, so she chooses to exert her dominance over Isabel and the other slaves. I suppose it is also possible to think that Madam Lockton is mean to Isabel as a way of venting her frustrations. Sometimes people come home angry at their bosses and go for a run or punch a pillow. Madam Lockton brands slaves and locks them in basements.