Here we have another example where the simple past-tense form of a verb serves as the past participle. This particular one is interesting in that Oliver Cowdery himself edited the printer’s manuscript: the change from slew to slain is in heavier, darker ink (just like Oliver’s editing in verse 21, where he changed supposeing to supposed ). Elsewhere, in the earliest text, there are two other examples of slew used as a past participle (in Alma 18:6 and Ether 8:6), but these were not edited to slain until the 1837 edition. For a complete discussion, see under past participle in volume 3.
Summary: In accord with the earliest textual evidence, we restore the original cases in the text where the simple past-tense form was used as the past participial form.