Here the original manuscript undoubtedly read “by the hands of those who slew them”, the 1830 reading as well as the corrected reading in 𝓟. Scribe 2’s correction appears to have been virtually immediate since there is no change in the level of ink flow for the supralinearly inserted plural s. In fact, the word hands is written at the end of the line in 𝓟, and scribe 2 may have simply run out of room trying to squeeze the word inline there. In other words, the singular hand may never have been at all intended. The critical text will maintain the plural hands in this passage.
Summary: Accept the plural hands in Mormon 6:15, the 1830 reading and the corrected reading in 𝓟.