As discussed under Alma 17:16, the original text allows for expressions of the form “to know of something” (in addition, of course, to the more frequent “to know something”). In each case, we follow the earliest textual sources. Here in Helaman 8:8, the 1840 edition omitted the of, probably accidentally. Perhaps the typesetter was influenced by the fact that earlier in the verse the verb know lacks the of: “and he knoweth as well all things which shall befall us”. The 1908 RLDS edition restored the of to the RLDS text.
There are other passages where a mixture of know and know of occurs within the same sentence, yet there has been no editorial tendency to eliminate such variation:
Summary: Maintain the occurrence of of after the second knoweth in Helaman 8:8 (“as he knoweth of our iniquities”) even though earlier in the verse there is no of after the first knoweth (“and he knoweth as well all things which shall befall us”).