Here the 1830 compositor originally set the word murderers correctly. But as the signature for this part of the text was being printed, the type near the end of several lines started to work loose, so the press was stopped in order to tighten up the type. In the process, the compositor reset some of the type; and while doing that he accidentally switched the order of the m and r at the beginning of the word murderers, thus creating the rather fantastic rumderers.
Janet Jenson, in her 1973 analysis of in-press changes in the 1830 edition, identified the typo rumderers as an initial state, but in actuality it is a “corrected” state. In-press changes in the 1830 edition of the Book of Mormon will be discussed more fully in volume 3, in the section covering the printing of that edition. See there for a list of initial and corrected states in the 1830 edition. For Jenson’s analysis, see her article “Variations Between Copies of the First Edition of the Book of Mormon”, Brigham Young University Studies 13/2 (1973): 214–222.
Summary: Maintain the word murderers in Mormon 2:10, which is what the 1830 compositor originally set.