The spelling massacreed here in 𝓟 suggests that this past participial form was pronounced /mæsßkrid/ rather than the standard /mæsßktd/ (𝓞 is not extant for the word). The Oxford English Dictionary specifies that by the 19th century the spelling massacree had become identified as illiterate but that this spelling could be found in both the 18th and 19th centuries. This spelling undoubtedly represents the pronunciation ending in /kri/ rather /kt/. The OED also lists the 17th-century spelling massacry, which most likely also stands for this pronunciation ending in /kri/ (much like other polysyllabic words ending in ry, such as angry, country, husbandry, idolatry, ministry, and revelry). The occurrence of massacreed here in Alma 48:24 may, of course, be an instance of dialectal overlay on the part of Joseph Smith and Oliver Cowdery.
Elsewhere the text has one other occurrence of this verb, and in that instance the word is spelled in the manuscripts according to the standard pronunciation:
This spelling suggests that the spelling massacreed in Alma 48:24 may actually stand for massacre + ed rather than massacree + d.
Another possibility here is that we have a case of variation in pronunciation, the nonstandard /mæsßkrid/ in Alma 48:24 but the standard /mæsßkt/ in Alma 49:7. Elsewhere the critical text has allowed such variation, as in the competition between drownded and drowned; for that pasttense form, drownded is allowed the one time it occurs in the manuscripts, despite the fact that drownded may represent a case of dialectal overlay (see the discussion under 1 Nephi 4:2). In the same way, we can have massacreed in Alma 48:24 but massacre in Alma 49:7.
Summary: Maintain in Alma 48:24 the manuscript spelling massacreed since there’s a good chance it represents the dialectal pronunciation /mæsßkrid/; on the other hand, the manuscript spelling massacre, representing the pronunciation /mæsßkt/, will be retained in Alma 49:7.