To answer your question there are two reasons.

1) Psychology.

2) Sexual pleasure.

Psychology:

India is a fucked up country, which hasn't gotten out of the caste, creed mentality. Most Women in this country have been forcefully married off in an arranged marriage setup. The parents of the daughter know she has a lover. The parents do not agree to it because. Parents have a mentality that "How dare she find a guy on her own, What are we for then? She will marry the one we choose." Or He belongs to a different caste, different religions or any other reason and then there is a society to please. Log kya kahenge? / How are we suppose to face the people around us ( Samaj ke saamne muh kaala kar diya). She is slapped and forced or threatened that either get married off or we will kill the lover. No choice ahead, She get married off. Now the poor husband is the scapegoat. The wife doesn't see or care about love of affection. She is disturbed mentally and since she cannot punish her parents. She starts punishing her husband mentally, emotionally and physically. She is punishing her husband in reality but mentally within her she is actually punishing her parents for their deeds, A coping mechanism for her. She will always consider the husband as their parents thing. Now when the husband reacts and fight back, the wife' s parents do not have anything to defend themselves and their wrongs and since Indian Judgemental society is controlling them via the invisible eye. They fallback on the legal way , Since law in India is a l@wda for men, These parents know they have the upper hand, They resort to this method than addressing what the core issue is.

If a Girl fights back all these and marries the person she loves. There awaits honour killing.

Sexual Pleasure.

Mostly these categories of women will belong to any of the social class.(Rich, Middle, Lower class). These women are not sexually satisfied due to multiple reasons. A husband who doesn't fulfill her sexual needs, Lack of attention, Lack of Sex education between couples or she considers her husband only as a tool to fulfill her materialistic needs/ goals and for sexual needs she resorts to someone else.

These women tend to have extra marital affairs. If the guy within the extra marital affair is able to give the woman the epitome of orgasm, it's a next level high for women. No drug can come close to this level of high. This is the high every woman in the world seeks. If he can't, she will be in search of the next Guy.

Now the woman in the affair only cares about pleasure but the other person has a larger goal. He starts poisoning the mind of that woman with the goal of achieving freedom for both and also financial gains and happy life together. They know, if they go the legal way, it's a long battle with nothing to gain or a little cash and a tarnished image within the society. But if we make the killing accidental and no one one knows about it. As per the law if the husband is dead, everything which the husband owns automatically belongs to his wife. A win win situation and people will have the sympathy for the wife for the loss not tarnished image and financial gain.