Mormon tells us at the end of verse 15 that prophets have been slain because they testified of the coming of the Messiah. His first example of a prophet is Zenos. We learn that Zenos was slain for his beliefs in Helaman 8:19. There is no clear information telling us that Zenock was slain, but Mormon appears to imply that he was. This possibility might also be read into the beginning of Helaman 8:20, but the presence of other prophets who were not slain in the list makes this less certain. Mormon also places Zenos and Zenock as prophets of the Josephite lineage, and therefore ancestors of the Nephites. (Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon [Salt Lake City: Deseret Book Co., 1976], 260.)