Asalamualaikum wrt wb,
All praise be to Allah.
1.) In Islam it is not permissible to murder another innocent person, let alone one's husband. Life is sacred, if a person kills a single person, it is as if they killed all of humanity. One should treat their spouse with love and kindness, not murder, whatever the circumstances.
2.) Although some people would like to abolish Islam's allowing a man to take more than one wife (up to four), this is established in the Quran and Sunnah for the one who can afford it. It is not acceptable to quote heresay for evidence in Shariah. If someone claims that there is evidence for banning allowing a man to marry more than one wife, subject to conditions of equal treatment, please provide that evidence. In any case, if a man has two wives, and treats them both equally, and fulfills their huqooq, a woman should control her jealousy, as her husband is obeying Allah by seeking halal and not haram. Furthermore, what benefit will she get from breaking the marriage apart and separating the children from their father?
3.) A person should control their Ghiraah (jealousy) and not let the Shaitan cause a person to do evil by rationalizing it in the person's mind. Ghiraah should be for the rights of Allah, first and foremost. The Prophet, may peace and the blessings of Allah be upon him, stated that a woman should not seek to force her husband to divorce her sister (his other wife), since she will only obtain what has been written for her.
4.) If a woman does not wish for her husband to have a second wife, she should make it a condition in her marriage contract, in which case the condition will hold according to the Hanbali school. If she failed to do this, then she can seek to free herself from the marriage by arranging a Khula' (separation) from the husband by giving up part or all of her Mahr, or some agreed upon amount. However, in no instance can she resort to killing. This is a great evil from the Shaitan.
5.) Many people like to argue about the issue of a man taking more than one wife without providing any evidence from the authentic sources of Islam. I will not engage is such fruitless argumentation. However, let me suffice by relaying what the scholars mention: that the majority of men in the West engage in having more than wife, but they do not call her wife, they call her mistress, or girlfriend, or one night stand, or other shameful names. Islam seeks to establish a society based on purity, chastity, and responsibilty. It is ironic and hypocritical that societies that ban second wives allow adultery, nightclubs, pornography, prostitution, fornication, and the total exploitation of the woman's body. Is it better to have a girlfriend/prostitute, or a wife?
May Allah show us right guidance, and give us the strength to follow it.
And Allah knows best.