Here the earliest text has the present-tense murdereth rather than the expected past-tense murdered. The rest of the passage is in the past tense: durst (two times) and was punished (once). The 1830 typesetter replaced the present-tense murdereth with murdered; and when Joseph Smith came to edit the text here for the 1837 edition, he marked in 𝓟 the change in tense. One possibility is that murdereth could be an early transmission error for murdered. Note that the two preceding occurrences of the modal verb durst could be interpreted as being in either the past tense or in the present tense. In the Book of Mormon text, durst is usually in the past tense, but there are some instances in the present tense (see the discussion under Alma 1:33). In other words, murdereth may be a transmission error that entered the text because durst was interpreted as a present-tense verb form.
Despite this argument, there is one other passage dealing with legal judgment that switches between present-tense and past-tense verb forms in describing Nephite judicial practice as established by king Mosiah (here each original present-tense form is indicated with an arrow):
Each of the present-tense forms in this long passage have long since been edited to past-tense forms (in either the 1830 or 1837 edition); for discussion of this specific editing (as well as the emendation of wages to his wages), see under Alma 11:1–3. The present-tense forms occur so frequently in Alma 11:1–3 that it is obvious that the original text sometimes intentionally used the present tense to describe legal procedures, as if they were taking place in present time. Since the occurrence in 𝓟 of the present-tense murdereth for Alma 1:18 is consistent with the frequent present-tense usage in Alma 11:1–3, the critical text will restore the present-tense form in Alma 1:18. Nonetheless, there remains the possibility that murdereth is an error that was introduced into the text because the two preceding occurrences of durst were interpreted as present-tense conditional forms.
Summary: Restore the present-tense murdereth in Alma 1:18 since such usage is found in the similar description in Alma 11:1–3 of the Nephite legal system as established by Mosiah.