This sentiment is so close to Mosiah 29:17 (“For behold, how much iniquity doth one wicked king cause to be committed, yea, and what great destruction!”) that it seems likely that Moroni was reshaping the historical record to emphasize (or even create) parallels with Nephite history. He was retelling the story, rather than recounting it verbatim, and may have been working from memory. (See commentary accompanying Ether 5:1.) In the ancient world, such a practice would not be misrepresenting history but rather accurately representing its cyclical nature.