The image here is one of vengeance, and the particular imagery is quite appalling. It should not be taken literally, but rather than the intents of the oppressors of Israel will be turned back upon them. It is the literary equivalent of the "those that live by the sword shall die by the sword." Those who wreak such destruction will themselves be destroyed.
The ending is stamped with the imprimatur of the author. With his name the Lord seals the promise.
Redaction analysis: Jacob later (2 Nephi 6:16-17) cites some of these same verses without specific reference (2 Nephi 6:14"according to the words of the prophet"). Interestingly, one verse is not given precisely the same way: