You bully someone because you get a sense of pleasure and satisfaction from hurting and putting down another human being.
I disagree. A minority of bullies do it because of their sadism. For the majority, it's to prove that they worth more that the person they are bullying. Whatever because of a personal insecurity, social insecurity, in an attempt to overcome their own suffering, or whatever, there's many reason behind this.
How do you know it is not both?
Because of the way she reacted during the dinner. Both when she was explicitly jealous of her sister, and when she desperately tried to voice her own opinion, after the moment where Elisabeth tricked the MC.
She had no assurance at all, which imply that she isn't used to enforce her position, and that she never was effectively in a position where she had to use her own authority, like a sadist would be. And it's not due to the fact that she was facing her mother. Elisabeth had absolutely no problem to convince her. In fact she didn't even had to convince her, she simply voiced her opinion. This while Isabella was just trying to do it.
She's able to humiliate Anja, Heather and probably some others, but only because their are either submissive (Anja, and the MC in the submissive route), fear her (Heather), or have other motive and play along (the MC in the dominant route). If she was 10 years younger, sadism could be a possibility, but she's too old now. If she was really sadist, she would had already acquired the required authority.
There's also her frustration when Elisabeth won and the MC had to wear the chastity belt. Frustration that she later expressed by putting Heather in her cage ; she said it herself, Heather is inside only because she failed to put the MC inside. And this clearly isn't sadism, just bullying. A sadist would have admitted her defeat, enjoyed the suffering of the MC, and waited another occasion.
Finally, there's what is probably the best demonstration, the comparison with Elisabeth. At the pool, she made the MC focus on his crotch and the feeling coming from it, which will inevitably lead to an impossible erection. The fact that she did it to later enforce her gentle side, by covering the MC, don't change the sadism of this move. In fact it's the opposite, it make it more visible ; either you obey, and I'll not make you suffer more that needed, or you don't, and I'll make you live in hell.
At the opposite, Isabella did all that she can to prevent the said erection. Would she be effectively sadist, would she had really wanted to make the MC suffer, that she would have forced him to look at her when she was dressing. But no, instead of leading him to an impossible erection and all the implied suffering, she made him stay face to the ground. How a real sadist could have missed such opportunity ? Especially since she hate the MC ?
On a side note, the more I think about it, the more I feel that she's a total sub. It's not really hate that she feel against the MC (and more generally males), but fear. The fear that he can make her submit for sex, while she was raised to be a dominant woman.
The way she so nonchalantly strings words together to form insults of a maximum emotional impact is a thing of great beauty and requires a great deal of craftsmanship and intelligence.
Perhaps is it because I'm not a native English speaker, or perhaps because I'm not submissive, but her insult always made me laugh more than anything else. They tend to remember me the tantrum of my children, a good try, but it don't touch the target.
But as I said, I'm not submissive, I play this game because I like the story, not because I like the context in which the story is. Therefore, the target isn't at the same place for me.
You do not reach such a high level of skill in humiliation and degradation without understanding perfectly what you are doing.
Which doesn't mean that you are necessarily sadist. In my own language I turn this into an art, but I'm a gentle guy, as long as you don't touch to someone I care for. I can kill you with words, and that what I do, I don't want to make you suffer, I don't want to humiliate you, I kill you.
Which mean that Isabella can effectively be sadist, be she also can be submissive like I said above. In which case, she didn't mastered the art of insult to humiliate the other, but to protect herself. Seeing her will remember you what she said, therefore you'll avoid it as much as possible, to not suffer uselessly.
But well, once again it's just my reading of the story and characters. I can be right, as well as I can be wrong.