Here the original manuscript undoubtedly read “those secret murders”, the 1830 reading and the corrected reading in 𝓟. Oliver Cowdery initially wrote “their secret murders” in 𝓟, undoubtedly in anticipation of the following their (“in their hearts”). For further discussion of this tendency on Oliver’s part, see under 3 Nephi 2:12 (where the was replaced by their) and under 3 Nephi 4:24 (where his was replaced by their).
Summary: Maintain the determiner those before secret murders in 3 Nephi 5:5, the reading of the earliest textual sources.