JOEL - To the best of my knowledge even though some LDS men were being killed off, it was not one of the primary reasons for the practice of polygamy. The way I understand it, the law of plural marriage was revealed to Joseph Smith around 1831 and the revelation was given to him under strict commandment from God to live it. At the time the doctrine was announced to the membership of the church in 1852, Orson Pratt explained, "the constitution gives the privilege to all the inhabitants of this country, of the free exercise of their religious notions, and the freedom of their faith, and the practice of it." In their opinion this included the practice of plural marriage, and an Illinois state law, prohibiting multiple marriages, was in their opinion in direct conflict with the Constitution. At the time there were no federal laws against it, until in 1862 when Abraham Lincoln signed the Morrill Bill, which was the first federal law against polygamy.
Pratt also said that, "marriage was ordained of God as the channel for spirits to acquire mortal bodies and that through plural marriage worthy priesthood holders could raise up a numerous righteous posterity unto the Lord." (Millennial Star, 1853). So, the primary reason was to help as many spirits as possible to obtain their bodies, be raised in good families, and to build up the kingdom of God on earth. The best way to do that and get it started quickly with only the few worthy saints that were available at the time was to live the law of plural marriage. Like it says in Jacob 2:27-30, "For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things." The prophets of old , such as David, Solomon, and Moses were commanded to do it. The restoration of the Gospel in this dispensation required that all doctrines and ordinances be restored, including that of plural marriage.
Of course only a small percentage of worthy priesthood holders were allowed to practice plural marriage at the beginning. Then later, it became evident that the laws of the land were not going to allow it. Persecution form other people and the government made it necessary for God to command the saints to discontinue the practice. In 1888 President Woodruff received a revelation and a vision from God showing him what would happen if they continued in plural marriage. He saw that the Church would lose all the temples; the First Presidency, Twelve Apostles, and all heads of families would be imprisoned; and that all personal property would be confiscated. This obviously would not help the church move forward and grow. This is when the Manifesto was presented to the church and the world, telling all members "to refrain from contracting any marriage forbidden by the law of the land."(Official Declaration -1).