Moroni declares that he is citing scripture. The only clear reference we have for this scripture is Romans 12:19. However, that scripture supplies only the parallel phrase “vengeance is mine… I will repay.” That part of the phrase was also found in Mormon’s text in Mormon 3:15.
The larger context given here, and specifically cited as scripture, is not part of our current set of scripture. It would appear that Moroni, and probably Mormon earlier, were citing a scripture from the plates of Nephi that was never copied into Mormon’s text.