How do Jehovah's Witnesses understand the essential nature of hell, and how does it differ from that of Seventh-day Adventists?

Unlike Seventh Day Adventists, Jehovah’s Witnesses believe there is no place of eternal torment. Like Seventh Day Adventists they believe that the dead are unconscious and know nothing (soul sleep). They both believe in a final and everlasting destruction of the wicked (soul annihilation).

Jehovah’s Witnesses quote 1 John 4:8 which says, “God is love” and ask, “Would a loving God really torment people forever?” They say Sheol and Hades refer not to a place of torment but to the common grave of mankind. Good people as well as bad people go to the Bible hell. Gehenna was a valley outside Jerusalem. It was used as a symbol of everlasting death. Here is a partial quote from their book ‘You Can Live Forever in Paradise on Earth - What Kind of Place Is Hell?’ (Chapter 9):

17 What, then, is “the lake of fire” mentioned in the Bible book of Revelation? It has a meaning similar to that of Gehenna. It means not conscious torment but everlasting death, or destruction. Notice how the Bible itself says this at Revelation 20:14: “And death and Hades [hell, King James Version and Douay Version] were hurled into the lake of fire. This means the second death, the lake of fire.” Yes, the lake of fire means “second death,” the death from which there is no resurrection. It is evident that this “lake” is a symbol, because death and hell (Hades) are thrown into it. Death and hell cannot literally be burned. But they can, and will, be done away with, or destroyed.

18 Since those who are thrown into “the lake of fire” go into “second death” from which there is no resurrection, they are, so to speak, jailed forever in death. They remain in death as though in the custody of jailers for all eternity. The wicked, of course, are not literally tormented because, as we have seen, when a person is dead he is completely out of existence. He is not conscious of anything.

Source: https://wol.jw.org/en/wol/d/r1/lp-e/1101989099 (Emphasis mine)

Unlike Jehovah’s Witnesses, Seventh Day Adventists believe there will be a literal, but temporary, place of torment at the end of the millennial reign of Christ. Here is an overview of their beliefs:

● The Bible teaches that the dead remain in their graves and there is no spirit, nor soul, which goes to Heaven (or hell) when we die. (Soul sleep)

● The Bible teaches that there is currently no such place as hell, purgatory, nor limbo. But there will be a literal place of torment after the 1,000 years only not eternal.

● The Bible teaches that, when the Lake of Fire becomes a reality (at the end of time), the wicked will not burn forever but will be completely destroyed. (Soul annihilation)

● The Bible teaches that when the unrighteous and Satan and his angels try to capture the Holy City, that Fire from Heaven will rain down upon them and create the Lake of Fire to destroy them all and burn them to ashes.

● The Bible teaches that then the earth, itself, will be destroyed by the Lake of Fire and that the righteous will watch from within the Holy City as God creates the Earth Made New. This earth will be destroyed then will be a new earth.

This link gives access to the fundamental beliefs of Seventh Day Adventists: https://www.adventist.org/beliefs/fundamental-beliefs/

In conclusion, Jehovah’s Witnesses and Seventh Day Adventists believe the dead know nothing and that they do not go into a place of torment (hell). Rather, their bodies go into the grave to await the resurrection and judgment.

Where they differ is with regard to the Lake of Fire. Jehovah’s Witnesses say it is not a literal place but is a symbol of “the second death” from which there is no resurrection.

Seventh Day Adventists believe there will be a literal place of torment at the end of the thousand years, but it will not be eternal. At the “end of time” the Lake of Fire becomes a reality. The wicked will not burn forever but will be completely destroyed.