Oliver Cowdery added the word behold here when he proofed 𝓟 against 𝓞. Since this passage could read with or without the behold, it was undoubtedly in 𝓞. There would have been no motivation for Oliver to have consciously added the behold here except that it was in 𝓞.
Summary: Accept in Alma 5:57 Oliver Cowdery’s corrected reading in 𝓟: “and behold their names shall be blotted out”.