I am religious and I believe in hell and heaven. I read many metaphors about hell but the closest metaphor I found in my religion is 'regret'. The regret of all bad acts someone does in his/her life and the regret of good things you could do. As you die, your soul left your body And so, I don't think what your soul is going to experience after that is materialist. So, I don't think burning, physical pain and so, .. is an option.
My theory in the ending is that the people that were sent to hell were given a test to repent their sins and if…
I have the same theory with a little diff. When someone gets the hell message from the angel(!) it is the beginning of their test. 'Jung Jin Soo' was acted right whole his life but at the end of his time, because he got confused between his free will and his destiny, did sin and killed that man(murderer) with the cooperation of the detective's daughter. Also, that "Hwasalchok member" in the last episode. But we did not see any sin from "Park Jung Ja" in her past or after she got the message. That's why we saw her reborn.