The way a lot of it works out in the bible, is that when you die right now. you die. There is no afterlife, you're in the grave. thats it. but then during the end times mentioned in 20:13-15, ALL the dead people are resurrected. And then the good ones get to stay alive and the bad ones go back to being dead. Thus a second death. So there is no "afterlife" then either... you are either eternally alive after armageddon, or eternally dead.

Being dead is basically just being non-existant. Fully unaware of anything. Which if you are to believe the bible, that means you're cut off from god entirely, who despite having just murdered basically everyone, is supposedly pure love and joy. So the end result is living in eternal bliss that is so great that you can't even fathom, or being cut off from that and never even having a possibility of anything. FOMO of god is the ultimate punishment.

I'm not even a religious person and the existential dread of "if it's truly nothingness after I die, i won't even have a way to experience the nothingness, everything will just stop" is enough to keep me awake some nights. So I can see how eternal nothingness was enough for the original authors to be considered horrifying consequence for not being religious enough, without having to resort to eternal physical torture.