Marrying already married women
It is forbidden to marry already married women within Islam, but it is allowed if they are captured. It does not matter if the womans husband is still alive - the woman is the male Muslims possession. This was explicitly stated as Mohammads companions had issues with this:
Sunan Abi Dawud 2155
Abu Sa'id Al Khudri said "The Apostle of Allaah(ﷺ) sent a military expedition to Awtas on the occasion of the battle of Hunain. They met their enemy and fought with them. They defeated them and took them captives. Some of the Companions of Apostle of Allaah (ﷺ) were reluctant to have relations with the female captives because of their pagan husbands. So, Allaah the exalted sent down the Qur'anic verse "And all married women (are forbidden) unto you save those (captives) whom your right hand posses." This is to say that they are lawful for them when they complete their waiting period.
Quran 4:22-24 specifically talks about this (with surrounding verses 4:22 and 4:23 providing context):
Quran 4:22-24
[22] Do not marry former wives of your fathers—except what was done previously. It was indeed a shameful, despicable, and evil practice.
[23] ˹Also˺ forbidden to you for marriage are your mothers, your daughters, your sisters, your paternal and maternal aunts, your brother's daughters, your sister's daughters, your foster-mothers, your foster-sisters, your mothers-in-law, your stepdaughters under your guardianship if you have consummated marriage with their mothers—but if you have not, then you can marry them—nor the wives of your own sons, nor two sisters together at the same time—except what was done previously. Surely Allah is All-Forgiving, Most Merciful.
[24] Also ˹forbidden are˺ married women—except ˹female˺ captives in your possession. This is Allah's commandment to you. Lawful to you are all beyond these—as long as you seek them with your wealth in a legal marriage, not in fornication. Give those you have consummated marriage with their due dowries. It is permissible to be mutually gracious regarding the set dowry. Surely Allah is All-Knowing, All-Wise.
Regardless of if a woman is married, if a Muslim takes her as his possession, then he can have relations with her. Note that the restriction is the waiting period, which is approximately 3 months.
Still not convinced? See common rebuttals
Rebuttal 1: This only applied in specific historical wartime contexts; Islam improved captives' status.
Response: Context doesn't change moral reprehensibility. Muslims may marry other married women as long as they have captured them, regardless of if the husband is alive.
Rebuttal 2: Islam forbids adultery
Response: Islam forbids adultery as long as the woman is not a slave. If she is a slave, she's fair game for the Muslim, regardless of if the husband is alive.
A Christians View
The Christian ethic stands in stark opposition to this. Jesus says the following:
Matthew 5:44
But I tell you, love your enemies and pray for those who persecute you
The idea of capturing another man's wife as a spoil of war is unthinkable. Christianity upholds the sanctity of the marriage bond as a lifelong commitment established by God, which cannot be dissolved by conquest. A marriage is to have exactly one husband, and exactly one wife.
Mark 10:6-9
6 “But at the beginning of creation God ‘made them male and female.’
7 ‘For this reason a man will leave his father and mother and be united to his wife,
8 and the two will become one flesh.’So they are no longer two, but one flesh.
9 Therefore what God has joined together, let no one separate.”
It calls for protecting the vulnerable, not exploiting them.
Psalm 82:3-4
3 Give justice to the weak and the fatherless; maintain the right of the afflicted and the destitute.
4 Rescue the weak and the needy; deliver them from the hand of the wicked.