In this paragraph Nephi further opens his heart, and makes it clear that it was anger that had "beset" him, as we have read in vv. 17-19. That was the "iniquity," the "sin," that caused his flesh to "waste away," and opened his heart to the adversary, to destroy his peace.
Nephi is absolutely correct in his estimate of the effects of paroxysms of anger on the body and mind of those who yield to it. I may state that I remember from the early years of my life an estimable lady, a good, faithful wife and the mother of beautiful children. At intervals she suffered outbursts of wrath to such a degree, that doctors told her that, unless she conquered those attacks, they would be her death. That is just what happened. She passed away after such an unreasonable outburst of passion.
I do not believe that Nephi suffered in a similar manner in any degree. I am rather inclined to the belief that he composed this song for the benefit of his brothers. They were angry with him. He may also have felt anger. They refused to listen to his sermons. Hence he may have hoped to reach them with a piece of poetry, as the Prophet Nathan had reached King David with a rebuke. Nathan had a little composition which he humbly submitted to the criticism of the king, the greatest writer of fiction at the time, in Israel. And it was not until the royal poet had given his opinion of the conduct of the hero of Nathan's story that the Prophet let the thunderbolt fall: "Thou art the man!" (2 Sam. 12:1-14) In the case of David it had the desired effect.
Anger, like fire, may be good, if kept under proper control. And some commentators tell us that is what the Apostle had in mind when he wrote to the Ephesians (4:26), "Be ye angry and sin not." Perhaps it is. I am not prepared to say it is not. But I have a suspicion that the negation belongs to both clauses of the sentence, and that the true meaning is, "Be [not] angry, and sin not." For, in the same chapter he says: "Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all bitterness." (v. 31)