As discussed under 2 Nephi 1:27, the object pronoun form him when acting as the subject complement has been frequently edited to the subject pronoun form he, in accord with the prescriptions of standard English grammar. (For further discussion of this kind of editing, see under subject complement in volume 3.) Here in Alma 1:8, the 1837 edition also changed the relative pronoun that to who, which is permissible from an editing point of view. There are 84 occurrences of he that in the original text, but only a single occurrence of he who (in 2 Nephi 24:6, an Isaiah quote). In the current LDS text, the imbalance remains, with 87 occurrences of he that and 10 of he who. Most of the current examples of he who were originally he which. The vast majority of cases of original he that have remained unchanged. The that was also retained in the following two cases where him was edited to he:
There is one instance of it that in 𝓟 that Joseph Smith edited to he who, but the 1837 edition ended up retaining the original that (yet making the change from it to he):
More generally, there are other places where Joseph changed that to who. For some examples, see the discussion under Mosiah 4:7. For a general discussion of Joseph’s uneven editing of that to who, see under which in volume 3. The critical text will in each case restore the original relative pronoun unless there is clear evidence of some kind of error or conscious emendation in the text.
Summary: Restore in Alma 1:8 the original object pronoun form him and the relative pronoun that (“it was him that was an instrument in the hands of God”).