The Book of Mormon text consistently reads evil here, yet evil is not the direct object for the verb phrase hast spoken. Instead, the relative pronoun which is the direct object. The word evil is actually functioning as an adverb here and is equivalent to evilly. The Oxford English Dictionary explains that evil is historically an adverb in the phrase “to speak evil of” (see definition 1b under the adverb evil ). The King James Bible has four instances where evil is clearly adverbial rather than nominal:
In all these instance, evil precedes the verb form spoken, thus assuring an adverbial interpretation for evil.
Such an adverbial interpretation of evil seems required in Mosiah 17:8. Here the critical text will retain the correct adverbial form evil, but with the understanding that it means ‘evilly’ and does not function as the direct object for hast spoken. In the original Book of Mormon text, the verb speak frequently takes adverb forms without the -ly ending, such as “I have spoken plain” (2 Nephi 25:20) and “it speaketh harsh against sin” (2 Nephi 33:5). See those passages for further discussion.
Summary: Maintain in Mosiah 17:8 the adverbial use of evil in the relative clause “which thou hast spoken evil concerning me and my people”; such usage without the -ly ending is fairly common in the original text of the Book of Mormon.