With that out of the way, a lot of the thoughts that follow come from Brant Gardner's Second Witness series. Gardner and others he referenced provide some very interesting context surrounding the historical setting of 1 Nephi. "The fall of Samaria and the northern kingdom [of Israel] ... [caused] the relocation of a large number of refugees from the northern kingdom into Jerusalem. Among those refugees were likely to have been Lehi's great-grandparents....The destruction of Israel was a caution to Judah, but one that Judah did not heed. Mistakenly assuming that his allies were more powerful than they really were, Hezekiah also rebelled against the Assyrian domination...."
Political Background
"King Josiah reigned from 639 to 608 B.C. After his death, Egypt placed Jehoiakim on the throne as a vassal king who, after Babylon drove Egypt out of Canaan, shifted his allegiance to Nebuchadnezzar but later revolted at his first opportunity. Nebuchadnezzar deported Jehoiakim's son, Jehoiachin, and installed Zedekiah as a vassal regent. Lehi received his call as a prophet under Zedekiah's reign."
Religious Background
King Josiah changed the religion of Israel in 623 B.C. removing all manner of idolatrous items from the temple, eliminating Canaanite practices; all done with the finding of an old law book (Deuteronomy) in the temple (similar to reforms made earlier by Hezekiah).
According to some historians, Lehi and his family fit into the category of people who left Jerusalem and did not agree with the reforms made. The Book of Mormon represents Israelite religion in the pre-exilic (before they were exiled by the Babylonians) period. Many themes of the Book of Mormon emphasize some elements of the pre-reform religion, though not opposed to all the reforms.
Possible Explanations for Specific Incidents
Understanding what was going on at the time allows for some interesting assumptions to be drawn as to why Laman and Lemuel were such murmurers. During this time, there were not really any agnostic Israelites. Gardner references that religion was not a separable component of life; religion was just how they lived their lives. So to assume that Laman and Lemuel were just not that religious probably isn't that realistic. If Lehi clung to the pre-reform beliefs, then it's likely Laman and Lemuel could have accepted the reforms made and thus saw no reason to leave Jerusalem or be unhappy with the way the Jews were living.
Laman and Lemuel are almost always referenced as a single entity, as if they had no individualistic thoughts or beliefs on their own. This serves to simplify Nephi's story telling, but also makes it easy to reduce them to one-dimensional beings having no emotional or personal depth.
Laman was the birthright son. Nephi's constant efforts to take the lead surely bugged them to no ends. "A prophet is not without honor, but in his own country." Their persecution of Nephi is not excusable, but how hard must it have been to see this taking place and having it occur to you?
The constant reminder that Jerusalem would be destroyed would also have been difficult to accept. Jerusalem had been attacked several times in recent history and hadn't fallen yet. It would have been easy to attribute the reason for it's continued existence as its people's righteousness. Why would we need to leave if God has been blessing the city for its righteousness? Surely there were other indications to the city's wickedness, but this would have been a pretty strong argument for the other side.
This list is by no means exhaustive. I think it's helpful to try to see things through someone else's eyes. Even if those people are proven wrong, it's helpful to see what it might have been that caused them to be so obstinately blind. Any other instances with Laman and Lemuel you can think of that could be viewed through a different lens?
