Then how do you understand Revelation 20:15, Revelation 14:10-11, and Matthew 25:41,46?
For what is eternally punishing (Matthew 25:46, Revelation 14:10-11) about the future, "Gehenna" hell (Luke 12:5, Greek: G1067) is fire eternally burning the physical body, and worms eternally eating the physical body (Mark 9:46, Isaiah 66:24). The physical bodies of non-Christians in Gehenna need not be exactly like people have now, which do not regenerate parts of themselves if those parts are burned or eaten. For before non-Christians are cast into the Gehenna hell (also called the lake of fire), they will be physically resurrected (Revelation 20:12-15, John 5:29b). And their new, physical resurrection bodies could eternally regenerate parts of themselves whenever those parts are burned or eaten. But then the regenerated parts could be burned or eaten again, only to regenerate again, only to be burned or eaten again, and so on, forever: an everlasting suffering (Revelation 14:10-11).
In Gehenna the fire will never go out (Mark 9:46). It will never run out of fuel, but will continue to punish non-Christians forever (Matthew 25:41,46, Revelation 14:10-11, Revelation 20:10,15). The fact that the fire will already be burning before the physical resurrection bodies of non-Christians are cast into it (Matthew 25:41, Revelation 20:15) means that their bodies will not be the fire's fuel. The fire will have its own source of fuel by which it will burn/punish non-Christians forever (Revelation 14:10-11, Revelation 20:10,15, Matthew 25:41,46, Mark 9:45-46).
Revelation 20:15
And whosoever was not found written in the book of life was cast into the lake of fire.
Does that verse say that those cast into the lake of fire are alive?
No, it does not, so from that verse alone we cannot assume that they are nor are not.
For that matter is the lake of fire literal or figurative?
God uses figures of speech abundantly in scripture so to ignore them is counterproductive.
Revelation 14:10-11
10 The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
I am not claiming to understand fully what these verses mean but consider the following.
Is it literal wine? Is is grape wine or something else? What does a cup have to do with eternal punishment?
Revelation 14:7-9
7 Saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters.
8 And there followed another angel, saying, Babylon is fallen, is fallen, that great city, because she made all nations drink of the wine of the wrath of her fornication.
9 And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
Who is going to drink of this cup?
a. those who were alive before the beast appears in the future
b. those who worship the beast and receive his mark on his forehead or in his hand
The beast is future not today so to extrapolate what is future into the present let alone into the past is not justifiable.
Matthew 25:41,46
41 Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
46 And these shall go away into everlasting punishment: but the righteous into life eternal.
Will those referred to in verse 41 be dead or alive when they are in the everlasting fire?
verse 46 tells us that they will be dead.
Where do the righteous go?
into life internal.
life eternal is set in contrast with the "these" by the word "but" which sets the second phrase in contrast to the first phrase.
Thus in contrast to life eternal we have death eternal which is sufficient everlasting punishment for they will die the second death knowing the goodness of God that they rejected and abhorred.
God is love and love is not cruel. God is merciful thus He does not torture people. He simply lets their own evil be the source of their own self destruction.
They rejected the love and light and life that God offered so they reap the results of their choices
To accuse God of cruelty is not right