Here Joseph Smith supralinearly inserted the subject pronoun he as if he intended to emend the relative clause “which had been slain” to “whom he had slain”. Such a change would actually have been consistent with the preceding text that speaks of Alma himself as having slain and driven back the Lamanites and cleared the ground of their dead bodies: “until he slew and drave them back and thus he cleared the ground or rather the bank … throwing the bodies of the Lamanites … into the waters of Sidon”. But after inserting the pronoun he, Joseph apparently decided that he should not make that change. Thus he left the passive auxiliary form been in 𝓟, but he neglected to cross out his inserted he and correct whom to who. The 1837 edition maintained the original passive in this relative clause. That edition did make the grammatical change of the relative pronoun which to the subject form who (not the object form whom, as marked in 𝓟). For a general discussion of the editing of which to who(m), see under which in volume 3.
Summary: Maintain in Alma 2:34 the original passive form of the relative clause “which had been slain”.