One wonders if the infinitive phrase here isn’t missing the preposition from; that is, perhaps the original text read “to destroy them from off the face of the earth”. The original manuscript isn’t extant here, but it is quite possible that it had the from and Oliver Cowdery accidentally omitted it as he copied the text from 𝓞 into 𝓟. There are three instances in 𝓟 where Oliver momentarily omitted from, although not from this particular expression:
The second instance is similar to the case here in Helaman 6:20 in that we have an isolated, optional occurrence of from. To be sure, there are quite a few instances of from before in the text (21 of them, of which nine involve the verb flee, as in Mormon 4:20: “and they fled again from before them”). Even so, the from is not required for that expression; for instance, there are 27 instances in the text of the expression “to flee before someone”, that is, without the from, as in Mosiah 19:21: “and they fled before them”.
Elsewhere the text definitely supports the preposition from in the expression “to destroy from off the face of the earth/ land”:
The King James Bible also supports the use of from with off in this expression:
Additional biblical support for the use of from off can be found in Joshua 23:15: “until he have destroyed you from off this good land”. The expression with from also occurs in a revelation given to Joseph Smith during the general time period that the Book of Mormon was translated (in this case, in March 1829):
These many examples provide considerable evidence for emending Helaman 6:20 to read “to destroy them from off the face of the earth”.
Nonetheless, it should be pointed out that originally the word off meant ‘from’; in fact, the preposition of derives from off. Thus from a historical point of view, the sense of ‘from’ is already present in off and the from is not necessary. Or one could equivalently argue that we could have from without the off in this expression. In fact, there is one instance of that form in the King James Bible, in Genesis 6:7: “I will destroy man whom I have created from the face of the earth”.
Since the earliest reading without the from will work, the critical text will maintain that reading here in Helaman 6:20, although it is quite possible that an original from was lost during the early transmission of the text. Examples in earlier English follow the biblically styled expression “destroyed from off the face of the earth/land”, but by the 1800s there are examples without the from, as in Thomas D’Arcy McGee (1861): “they ought to be destroyed off the face of the earth” (listed on Literature Online ). It is also worth noting that the modern expression “to be wiped off the face of the earth/land” typically lacks the from. So at least the modernizing tendency has been to remove the from in the expression “to be destroyed from off the face of the earth/land”.
Summary: Maintain in Helaman 6:20 the earliest reading without the from in “to destroy them off the face of the earth”; there is indirect support in Genesis 6:7 for a shorter reading without both from and off; elsewhere the scriptures prefer from off in this expression (five times in the King James Bible, ten times in the Book of Mormon, and once in the Doctrine and Covenants), which suggests that from may have been lost during the early transmission of the text for this one exception.