As explained under Alma 46:12, there are six examples in the original text (including the example here in Ether 14:17) where the word thereof appears to have no referent. The editors for the 1920 LDS edition apparently made that assumption in this passage since they deleted the thereof. This may have been motivated by the fact that their copytext here actually read “and he did slay both women and children”, and it may have seemed odd to the editors that the text referred to cities of only women and children. Of course, if the text is emended to read “and he did slay both men women and children” (as explained above in an earlier discussion), then the use of the thereof at the end of the following clause would make better sense.
Nonetheless, we should note that elsewhere in the Book of Mormon (we exclude the six examples without any apparent referent) the word thereof does not refer to people, only to things (154 times). Most of these instances of thereof mean ‘of it’, but a number of them mean ‘of them’— that is, they refer to plural things (but not to people), as in the following sampling:
The King James Bible has 908 examples of thereof, and every one of those mean ‘of it’, never ‘of them’. The same holds for the examples cited under thereof in the Oxford English Dictionary. If we refuse to accept people as a possible referent for the thereof, then we would be forced to interpret the original thereof in Ether 14:17 as having no referent, like the other examples listed under Alma 46:12. In any case, the thereof is fully intended here in Ether 14:17 and will be restored in the critical text.
Summary: Restore in Ether 14:17 the thereof that was deleted in the 1920 LDS edition, probably because there appeared to be no appropriate referent for it in the preceding clause (“and he did slay both women and children”); however, if the original thereof refers to people (such as “both men women and children”), we would have a unique use of thereof in the text.