Oh boy! Bible verses taken out of context! Let’s look into these, shall we?
Deuteronomy was about preparing Israel for the coming of Messiah. It’s not a command for today’s Christian people. Same with Exodus. They exist as historical record.
Psalm 137 is lamenting what Babylon did to Israel. It was an eye for an eye rhetoric, as the Babylonians had murdered their children. Again, Psalms aren’t prescriptions. They’re songs written by a Jewish king.
Mormons aren’t Christian. They reject the Trinity for a start and have additional scripture, just like Islam with their Qur’an or Jehovah’s witnesses with the Watchtower publications.