Maybe it doesn't say that. Maybe the warning is about Earth. Maybe forever means Man's offspring. Maybe the second death compares with the first death which was mankind in the story of Noah.
Someone has said that forever in the Bible means an age. Maybe God's will is for mankind to last the age of the Earth. Maybe the end of the age is when the sun goes out or explodes or whatever it is going to do. (I hear the universe will end too) If humankind does not care right for Earth then Earth will be hell for our offspring as it seems to me that for some their life is hell. Hell appears to be spreading.
Isa.45.18 "For thus sayeth the LORD that created the heavens; God himself that formed the earth and made it, he hath established it, he created it not in vain,
he formed it to be inhabited; I am the LORD; and there is none else". Ps.78.69 "-- like the
earth which he hath established
forever". Ps.115.16 "The heaven, even the heavens, are the LORD'S, but the earth he has
given to the children of men"; .Eph.3.21 "Unto him be glory in all the Churches by Jesus Christ throughout
all ages,
world without end. Amen" Rev.11.15 "--The kingdoms of
this world are become the kingdoms of our Lord and his Christ".
Since Ps.146.4 says "His breath goeth forth, he returneth to his earth, in that very day his
thoughts perish", and Job 3.17-19 says "There the wicked cease from troubling, and there the weary be at rest. there the prisoners rest together, they hear not the voice of the oppressor, the small and great are there, and the servant is free from his master", it is not likely that hell is where non believers go when they die.