In this verse we find the degree to which Laman and Lemuel had hardened their hearts against the things of the Spirit. Imagine the scene, where in the midst of doing violence with a staff upon their younger brothers, an angel appears. The mere appearance of the angel would seem to be sufficient to cause wonder and pause, but even greater than the appearance was the direct chastisement of Laman and Lemuel. To all of this wonderful (in the very old sense of "full of wonder") situation, Laman and Lemuel don't appear to discuss the angel, nor even remark upon the visitation. They merely continue their disbelief, citing reasons why the message had to be wrong. Perhaps they were attempting to show the message wrong so that they could deny the messenger. In any case, it is clear that they did deny the messenger, and his message.
There is no other indication that such an appearance had ever happened to either Laman or Lemuel. Their first experience with an angel is significantly different than Nephi's. Perhaps their lack of wonder at the appearance was a tacit admission that their father and brother had tapped into the spiritual realm. Perhaps they believed that it was possible, but not infallible. They were obviously able to justify this experience in a way so as to deny its real power.
When on my mission, my companion and I had the occasion to place a Book of Mormon with a man. A couple of weeks later there was a great thunderstorm, and there was a particular clap of thunder in the night that awoke my companion from his sleep. While I managed to sleep through it, my companion proclaimed it to have been the loudest clap of thunder he had ever heard.
A few days later we were checking up on our placement of the Book of Mormon, and the man invited us in. He said "I really need to talk to you. I read the Book of Mormon after you left, and couldn't put it down. I was really touched by it, but I just wasn't quite sure if it were true. I decided to kneel down and pray, and asked the Lord for help. I wanted some sign to tell me that the Book was true, but I knew that I shouldn't ask for a sign. Nevertheless, I needed some assurance. It was stormy outside, and so I thought that I would ask the Lord for some thunder, so I would know. I then realized that it would thunder anyway, so I asked to hear the loudest clap of thunder I had ever heard".
That was the thunder which awoke my companion. Unfortunately, as the man described his story to us, he ended by saying that it was raining anyway, and so he really couldn't be sure. He returned the Book of Mormon to us, and we said our goodbyes.
Laman and Lemuel are not the only ones who can stare the divine in the face, and deny it.
Socio-Cultural background: Nibley provides some cultural background on Laban and his fifty. "As to the garrison of fifty, it seems pitifully small for a great city. It would have been just as easy for the author of 1 Nephi to have said "fifty thousand,"and made it really impressive. Yet even the older brothers, though they wish to emphasize Laban's great power, mention only fifty (3:31), and it is Nephi in answering them who says that the Lord is "mightier than Laban and his fifty," and adds, "or even than his tens of thousands." (4:1). As a high military commander Laban would have his tens of thousands in the field, but such an array is of no concern to Laman and Lemuel: it is the "fifty" they must look out or, the regular, permanent garrison of Jerusalem. The number fifty suits perfectly with the Amarna picture where the military forces are always so surprisingly small and a garrison of thirty to eighty men is thought adequate even for big cities. It is strikingly vindicated in a letter of Nebuchadnezzar, Lehi's contemporary, wherein the great king orders: "As to the fifties who were under your command, those gone to the rear, or fugitives return to their ranks." Commenting on this, Offord says, "In these days it is interesting to note the indication here, that in the Babylonian army a platoon contained fifty men"; also, we might add, that it was called a "fifty," hence, "Laban and his fifty." (Nibley Lehi in the Desert 1950 pp. 111-112).
Redactive comment: Verse 31 ends chapter 3 in the modern edition, and separates this section from chapter 4's events. While there is a change of intent and speaker which allows this shift, it is all part of the same story, and was not so divided in the original edition of the Book of Mormon. The division of the story of the brass plates and Laban is a single story, and the break is somewhat arbitrarily placing a break in the middle of a continuous narrative.