Question: Why would an angel appear to Alma the Younger and the sons of Mosiah when they were going about “seeking to destroy the church and to lead astray the people of the Lord”? (Mosiah 27:10)
Answer: President Wilford Woodruff told of the Lord having a specific purpose in sending angels to the earth: “He sends angels to perform a work that cannot be performed only by the administration of an angel...The Lord has sent angels to men from the creation of the world, at different times...”
As for Alma the Younger and the sons of Mosiah, the Lord needed them to mend their ways in order that they might be called to perform a great service in His kingdom. The purpose of the angelic visitation was to “convince [Alma] of the power and authority of God, that the prayers of his servants might be answered according to their faith” (Mosiah 27:14).
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 145