The preposition of in the phrase “to take of their flocks” is difficult for modern readers, which may explain why the of was changed to off in the 1837 edition. Another possible change would have been to simply delete the of: “and began to slay them and to take their flocks”. (For an example of where the of was deleted, see under Mosiah 21:21.) Yet the use of of here in Mosiah 9:14 is correct. Originally in English, the preposition of and the adverb off were the same word; its meaning was essentially the same as the modern word from. Here in Mosiah 9:14, the of implies a partitive sense: not all their flocks were taken, only some of them. Of course, the verb began implies the same partitive sense.
The 1837 reading “to take off their flocks” does seem rather odd, which suggests that the 1837 change to off may have been accidental (it was not marked by Joseph Smith in the printer’s manuscript). Even if we interpret take off as meaning ‘to remove’, the language still seems strange. Elsewhere, the text has examples of “to take off X” with the meaning ‘to remove X’, but in these other cases the phrasal verb take off refers to removing something from off a person:
On the other hand, there are quite a few examples of “to take of X”. All of these have a partitive sense and thus agree in this respect with the original reading of Mosiah 9:14:
Thus the evidence is overwhelming that the original of is the correct reading in Mosiah 9:14. Based on the reading in 𝓟, the 1908 RLDS edition restored the preposition of to the RLDS text.
Summary: Restore the original partitive preposition of in Mosiah 9:14: “and began to slay them and to take of their flocks”.