The idea that either heaven or hell exists, is cultural within the Bible.  Jesus was born at a time when the Greek culture had had some influence on Jewish thinking - a time where there was some discussion between the Pharisees and Saducees as to the existence of an afterlife.

It's not too difficult to understand why the carrot of heaven and the stick of hell were attractive ideas at the time. The Jewish people were suffering, subdued by their Roman overlords.  Jesus message wasn't one of military revolution, but of a change of heart and mind, which, if enough people bought into it, would result in a far better world - heaven on earth if you like.  A world where Love ruled (The Kingdom of Heaven) was an ideal, a goal, something to be aspired to.  When Jesus spoke about it he used very figurative, metaphorical language, incorporating the prevailing ideas of the day, to give a sort of picture of the concept or the dream of a world where the wrongs of humanity were set to right.

A culture under oppression needed that sort of a goal - they needed someone to tell them how to go forward to achieve their 'Kingdom' without trying to rebel and achieve a military liberation from their opressors.  The idea of eternal life was appealing because, for the indiviual, there was a yearning for justice, so the idea that God would come along and punish the bad people and reward the good people was a message their ears really wanted to hear.  But the sad fact is that, when you look deeper than the soundbite, justice can't really work like that. People are partially good and partially bad and a crime once committed isn't negated by the fact that the criminal is punished.  So justice can't really set things to right in the way that the idea of final judgement implies.

Good can't succeed by setting everything to rights by hurting those who did wrong - past wrongs are still there in their effect on us and punishment, if it to have any purpose, has to be about rehabilitation and drawing out a recognition from the offender, and consequently a wish to change.  Punishment that goes on for ever and ever without any hope of that is therefore utter cruelty in itself - it turns the punisher into the evil one.