Whatever Laman and Lemuel might have justified to themselves about the lack of the hand of God in their enterprise was dramatically countered by the appearance of the angel. Angelic interference in the affairs of man are fairly uncommon, and this enterprise must have had critical importance for the plans of the Lord to allow for the angelic manifestation. It will not be the only time. However, note that the appearance of the angel does not change Laman and Lemuel's heart.
If the purpose of the anger were only to stop the beating, the Lord could have found other means. Nephi could have been made stronger, an earthquake could have come up... many things which could be explained in the natural realm could have happened. Since the angel did not have much of a lasting impact on Laman and Lemuel, perhaps the purpose of the angel was for Nephi, to strengthen his faith for the tasks to come.
Note that the angel speaks to Laman and Lemuel and clearly declares that Nephi is to be a ruler over them. It is unclear from Nephi's record up to this point whether he shared his vision with his brothers, or only with his father. If Nephi only shared the vision with his father, then Laman and Lemuel are learning of this prophecy for the first time. They surely were no more happy about Nephi's predicted rulership than were the sons of Israel about Joseph's prophesied ascendency.
It is also perhaps interesting to notice with our reader's foreknowledge, that Laman and Lemuel eventually effectively negate this prophecy once they separate from Nephi's band in the New World. The prophecy comes true, but only during the crossing through the wilderness and the ocean. Once in the New World, Laman and Lemuel choose separation from their kin rather than submit to Nephi's leadership.