Well, Lehi's family has finally made it to the Promised Land, and you would think the happily ever after would kick in here. Except that this is real life, and there's always another story to be told.
As long as Lehi lived, Laman and Lemuel showed some restraint. Eventually, however, Lehi did die, and after a few days, they began to complain again. You see, as the older brothers it should have been their birthright, Laman's specifically, to be the religious heads of the family. Unfortunately, due to their constant rebellions and attempts at murder in their travels, they had lost that right, and it passed to Nephi (I actually think it should have been passed to Sam, but he was always a humble man, and I think he understood that Nephi was a natural-born leader. It never actually mentions what happens there). Laman and Lemuel couldn't get over the fact that their younger brother was considered their rightful leader by most of the family, and eventually, their murderous habits popped up again. They decided they were going to kill Nephi.
As evidenced through most of the Books of Nephi (the first two sections of the Book of Mormon), they could have been stopped easily, but... come on, they had been threatening to kill him since day one, and their hatred for him was only growing. Before they could bring their plots to fruition, God warned Nephi in a dream to take everyone who would go with him and flee. Nephi and his followers escaped, and the only revenge left to Laman and Lemuel was to teach their children that Nephi was a liar and a thief (he'd taken with him the brass plates, the Liahona, and the sword of Laban). Eventually, two great nations grew up from these two groups. Nephi's descendants, as well as those of the people who left with him, were known as Nephites, and Laman and Lemuel's descendants were known as Lamanites.
One thing I find interesting about this story is that it specifically says "take all who will go with you". It's not mentioned, but I've always wondered if maybe some of Laman and Lemuel's children decided to go with them as well.
So, writing this post, I realized I haven't really explained the structure of The Book of Mormon. It's divided into 15 different sections, known as "books". The first two are the 1st and 2nd books of Nephi, and are written by Nephi. The next book, the book of Jacob, was written by his younger brother. The next three books, Enos, Jarom, and Omni, are written by the descendants of Jacob, and mostly they're a lot of "I'm the son of him, quick overview of history, pass to the next caretaker." Words of Mormon, the next book, is a brief insert by Mormon, one of the last Nephite prophets, who explains that for the rest of the book he will be abridging the history and teaching of his people (apparently the original record would be even bigger than the Bible). The books of Mosiah, Alma, Helaman, 3rd and 4th Nephi, and Mormon, all record the long history of the Nephites, as well as the teachings of their prophets. Ether is a record translated by King Mosiah of the people living in the Promised Land before Lehi's family, and the last book, Moroni, is written by Mormon's son, the very last, very lonely Nephite prophet. He survived the destruction of his people long enough to finish and hide the record so it wouldn't be destroyed by the Lamanites.
Fare thee well, friend!