Demons are the angels of the devil, spirits subject to him who work to lead people into evil. They persuade no one to do good, to believe in Christ, or to serve God; whatever persuades a person to do evil and deny Christ is of the devil, and his angels work the same way (Moro. 7:17). The Book of Mormon explains this drive: the devil fell from heaven and became miserable forever, so he seeks that all men might be miserable like unto himself (2 Ne. 2:18, 27); his angels share and execute that same misery-seeking purpose.
Those who die filthy and unrepentant are reckoned with the devil and his angels: they remain filthy still, become like him, and are shut out from God’s presence to dwell with the father of lies in misery (2 Ne. 9:9). They depart into the everlasting fire prepared for the devil and his angels, their torment a lake of fire and brimstone (2 Ne. 9:16; Mosiah 26:27). Jacob warns the people to awake from the slumber of death and loose themselves from the pains of hell so they do not become angels to the devil, cast into the lake of fire that is the second death (Jacob 3:11).
When people are slain in their iniquity, the devil laughs and his angels rejoice (3 Ne. 9:2). Samuel the Lamanite prophesies that in the day of their destruction the Nephites will say, “Behold, we are surrounded by demons, yea, we are encircled about by the angels of him who hath sought to destroy our souls” (Hel. 13:37). Their existence is part of the opposition in all things, the contrast of righteousness and wickedness, good and bad (2 Ne. 2:11).