In the excitement and thrill surrounding their victory over the Lamanites, the Nephite armies not only boasted in their strength but also began to swear oaths of vengeance against the Lamanites.
Mormon was utterly offended by such action because it was another evidence of their collective and individual spiritual emaciation. With this practice of oath-swearing they were blatantly disregarding the commandment of the resurrected Jesus to their ancestors (see verse 14; 3 Nephi 12:33-37).
It was not just this act of disobedience that repulsed Mormon, and undoubtedly offended God, but that by using the name of Deity and swearing by the heavens they were profaning and blaspheming God’s sacred name.
How hypocritical and profane to trample under foot the commandments of God and the words of his holy prophets, through wilful rebellion, and then swear by sacred names and means to suit one’s own carnal desires and wicked ambitions! This epitomizes one meaning of “take the name of God in vain.”