But Biblically though I thought God wanted it to just between 2 people? How does LDS justify it?
Justify what? Taking care of people?
If you're thinking of sexual relations, fine. The Old Testament didn't condemn men who lived up to their obligations, like Boaz in the case of Ruth, who was his second wife. Book of Ruth doesn't say so in just so many words, but it does refer to a principle found in Leviticus, regarding a duty to marry her, even though he was already married. But this topic is sociopathy, not sex.
Most men who would cheat on a marriage are in the frame of mind that it's not a marriage they are looking for. They also don't think they are doing anything great. Some men who do polygamy actually think they are good and great men, and deserve respect, and expect to be respected for their families. In some cases, that might actually be pretty true. In other cases, it might be very untrue. In my judgment, it is untrue when they demand a place in the mind of their followers/wives that is displacing the role of personal conscience, exerting power and other vices or evils rather than virtues in their dealings.
Of course, the standard LDS response would be "We don't do that anymore".
But the practical discussion to be made here is on the plane of social science, more than theology. Sociopathy is a specific term, though it is graded on a range of issues or characteristics. The core concept is a descriptor of a deranged sense of social responsibility. The less you care about others, and the more callously you mistreat them, the more "sociopath" you are. Doing it under a false flag of religion is just one more layer of evil.
So here is a counter example, from a hundred years ago, and more. My great-grandfather had three wives, maybe twenty five children. He was a respected man in Salt Lake City, with a high office in the Church, and a good carpenter and farmer. When one of his wives died, he was at her beside. She asked rather delicately, "Will you miss me?". His took her hand, and said, to be strictly correct on a sensitive subject, "We will miss you". His point was that he was not going to divide his family in principle. He and his wives had a certain shared unity and commitment.
Those folks worked together and went through everything in life together, on equal terms. He made himself a devoted servant to his wives and children. Did not lift himself up, so to speak. He was pretty far from being a sociopath. He was all about doing stuff for other people.