When a woman is with a man whom she does not deserve, she is likely to turn into a brat. She would take for granted what she has with a person beyond her merits, without remembering that she would not be able to get anywhere near that level on her own. Winnie Mandela was nowhere close to deserving Nelson Mandela, and she proved that through her behavior. Whereas Nelson Mandela's wife after her - Graca Machel, former head of Mozambique - was a much more appropriate choice of partners.
When a man is with a woman whom he does not deserve, he typically becomes abusive. He knows that he does not deserve the woman, so he reduces her value and beats her down in order to justify a relationship in which he is the head of someone whom he doesn't deserve. Typically he invents reasons to justify the abuse: that she's evil, that she's crazy, that she's arrogant, and further on along the same line. The real reason for his behavior is that he is with someone whom he knows to be better than him; and to justify such an arrangement he would invent any reasons he can to reduce her value and beat her down into a place in which he is in control.
In both cases the partner who doesn't deserve the other partner takes for granted a privileged state and acts as if that state belongs to them naturally. The worse partner becomes spoiled and appropriates what is not theirs without rewarding the better partner appropriately and while frequently beating the better partner down. We see this in any number of situations, and it is quite a guarantee that, when such are the dynamics, that is the behavior that we will see.
The only way to counteract these malignant dynamics is for the better partner to see what is going on and either stand up to the abuse or the brattish behavior, or else go for a partner who is closer to their own merits.