Here in his editing for the 1837 edition, Joseph Smith removed the archaic use of him after the verb repent. This kind of reflexive language (namely, “one repents one(self ) of something”) is fairly frequent in the Early Modern English of the King James Bible, occurring 13 times in the Old Testament and once in the New Testament. In the Old Testament most of the examples have the regular personal pronominal form after the verb repent, as in these examples:
Note in particular the examples that refer to repenting oneself of a particular evil, just like originally here in Ether 2:15. But in three of the biblical cases, we get the reflexive pronoun for this expression:
The original usage in Ether 2:15 (“the brother of Jared repented him”) is clearly intended and will therefore be restored in the critical text (even though this archaic usage involving the verb repent occurs nowhere else in the Book of Mormon text).
Summary: Restore in Ether 2:15 the original pronoun him in “the brother of Jared repented him of the evil which he had done”; such usage for the verb repent is found in the King James Bible but nowhere else in the Book of Mormon.