Many prophets have been killed after their missions were completed (see Matthew 23:37). The Lord will protect them until that mission is finished (see Mosiah 13:3; TPJS, 274, 328). He allows the prophet to be killed to “seal his testimony with his blood, that he might be honored and the wicked might be condemned” (D&C 136:39). We have no record of Nephite prophets being slain at this time, but apparently they were (Helaman 13:24).
During his earthly ministry, the Savior gave a chastisement to the scribes and Pharisees for slaying the prophets similar to the one Samuel gave to the Nephites (v. 25, see Matthew 23:30–31). Samuel said the Nephites were worse than the generation who killed the prophets because they labeled the true prophets as false prophets, and accepted the false prophets as true prophets (Helaman 13:26–28). Isaiah described a similar problem among the Israelites of his day: “That this is a rebellious people, lying children, children that will not hear the law of the Lord: Which say to the seers, See not; and to the prophets, Prophesy not unto us right things, speak unto us smooth things, prophesy deceits: Get you out of the way, turn aside out of the path, cause the Holy One of Israel to cease from before us” (Isaiah 30:9–11). Nephi gave a figuration description of “the very God of Israel do men trample under their feet, but I would speak in other words—they set him at naught, and hearken not to the voice of his counsels” (1 Nephi 19:7).
In desperation Samuel asks the Nephites how long they will be led by foolish and blind guides (instead of the prophets), and choose darkness rather than light (Helaman 13:29). The blood of the saints, and prophets are also saints (see Revelation 18:24), shall cry from the ground for vengeance, whether it be physical blood or spiritual rejection and trampling to death (see 2 Nephi 28:10; D&C 7:7).