Eternal torment is God’s torment or punishment (see D&C 19: 10–12). To harrow the soul is to break up the hardness of heart, as the farmer does in breaking up the hard clods of earth. In Alma’s case, all of his sins were brought to his memory (Alma 36:12). The pains of hell, as defined by Alma, is the torment of the mind when one realizes he has rebelled against God and not kept his commandments (v. 13; see also Jacob 3:11; Alma 26:13). Alma describes spiritual murder as the leading of souls to destruction (Alma 36:14). In the extreme, this type of murder is more serious than physical murder for which there is no “forgiveness in this world, nor in the world to come”(D&C 42:18). Spiritual murder, when willful, could be unpardonable and lead to becoming a son of perdition (see Alma 39:6). Alma may have realized this when he wished to become extinct (Alma 36:15). Alma experienced to some degree what Martin Harris did when he lost the 116 pages of Book of Mormon manuscript, and the Spirit withdrew from him (see D&C 19:20). Alma also experienced something similar to what Isaiah described which will take place at the Second Coming of Christ:
19 And they shall go into the holes of the rocks, and into the caves of the earth, for the fear of the Lord shall come upon them and the glory of his majesty shall smite them, when he ariseth to shake terribly the earth.
20 In that day a man shall cast his idols of silver, and his idols of gold, which he hath made for himself to worship, to the moles and to the bats;
21 To go into the clefts of the rocks, and into the tops of the ragged rocks, for the fear of the Lord shall come upon them and the majesty of his glory shall smite them, when he ariseth to shake terribly the earth. [2 Nephi 12:19–21; Isaiah 2:19–21; see also Revelation 6:12–17]
The extent of his suffering must have been horrendous when considered it lasted for three days and three nights.