The definite article the was deleted in the 1837 edition, but that deletion was not marked by Joseph Smith in his editing of the printer’s manuscript. The deletion is probably a typesetting error. There seems to be no particular grammatical or semantic motivation for removing the the. Elsewhere the text has only “most abominable”, although “the most abominable” would have been possible:
(For a third example of “most abominable”, but one that involves textual variation, see Ether 8:18.) The occurrence of “the most abominable” in 1 Nephi 13:26 is unique, but there is no substantive reason for removing the definite article.
Summary: Restore the definite article the in 1 Nephi 13:26 (“the most abominable”).