The Lord had given Mosiah a promise that none of his sons would be harmed among the Lamanites (Mosiah 28:7). With an armed man rushing to kill the helpless Ammon, the situation does not look good. No doubt the angel of the Lord was standing to protect Ammon and his royal converts. Abish doesn’t see the angel and Mormon is silent on the subject but this is often how the Lord protects his servants (See Num 22:21-35, 2 Kgs 6:13-17, and Matt 26:53).
“One of the functions of angels is to warn and protect mortals. The Lord whispered to David, ’There shall no evil befall thee, neither shall any plague come nigh thy dwelling. For he shall give his angels charge over thee, to keep thee in all thy ways. They shall bear thee up in their hands, lest thou dash thy foot against a stone’ (Ps. 91:10-12). The angel of the Lord’s presence saved Israel (Isa. 63:9). Daniel replied to the King: ’My God hath sent his angel, and hath shut the lions’ mouths, that they have not hurt me…’ (Dan. 6:22).” (Encyclopedia of Mormonism, p. 42)
Joseph Smith saw an angel of the Lord in vision protecting Brigham Young.
“I saw Elder Brigham Young standing in a strange land, in the far south and west, in a desert place, upon a rock in the midst of about a dozen men of color, who appeared hostile. He was preaching to them in their own tongue, and the angel of God standing above his head, with a drawn sword in his hand, protecting him, but he did not see it.” (Teachings of the Prophet Joseph Smith, p. 108)