Mormon is clearly the narrator here. Verse 6 introduces the story of Korihor, but verses 7 and 8 are an aside discussing the nature of Nephite law. Nephite law comes into play in the story, but would not have been a topic on the plates. Only Mormon’s understanding that his future audience would not know Nephite law explains why verses 7 and 8 are included.
The introduction to Korihor is that “there came a man into the land of Zarahemla, and he was Anti-Christ.” Due to modern perceptions of the term “anti-Christ”, it is too easy to erroneously read this as saying that he was an anti-Christ. That isn’t the meaning here. The meaning is that he is “against the prophecies which had been spoken by the prophets, concerning the coming of Christ.” His teachings were, therefore, very similar to the teachings of previous apostate Nephites, including Sherem and Nehor, the priests of Noah; they taught that one did not need to look forward to a coming Messiah.
Mormon adds verses 7 and 8 so that his readers will understand why Korihor was allowed to preach at all. There was no law against what a person might believe. It is interesting that Mormon should note that the law was supported by scripture. It was an interpretation of scripture, to be sure, but it was scripture, nevertheless. The reference is to Joshua 24:15: “choose you this day whom ye will serve.” The reference is to a choice between serving Jehovah or the gods of Egypt, but it was used to bolster any choice of religious belief that a Nephite might understand.