Laman and Lemuel were willing to go along with Nephi up until the point at which he failed. When the purchase of the brass plates failed, Laman and Lemuel not only lost their monetary inheritance, they lost their pride because they had been led by a younger brother who cited the Lord. Their dismay at his failure was also likely a self-justification which allowed them to deny that the hand of God had ever been in their enterprise. Their anger took a physical form.
It is probably appropriate to note here that such physical abuse was not only not appropriate in this case, but is not appropriate in any relationship. It is perhaps easier to see in the case of Nephi that the abuse was uncalled for, because we have sympathies with Nephi. Nevertheless, all too many are caught up in the moment of anger, and turn to physical domination and abuse to hold their point. Note that Laman and Lemuel were not hesitant to turn such physical abuse on a family member. Modern man (and woman) is no less prone to inappropriately use force, either emotional or physical, to attempt to place themselves in a superior position.