We Buddhists believe that a life, or to be precise, a soul, does not really die, but rotates in the wheel of different formats. This cycle is called samsara. There are six realms in samsara, which can be divided into three higher realms and three lower realms. The three higher realms consist of the heaven realm, the Asura realm and the human realm that where we are. The three lower realms consist of the hell realm, the animal realm and the hungry ghost realm.
It’s a wheel and at the same time it’s a hierarchical system, which means the higher you are, the more power and enjoyment you have. Contrarily, the lower you are, the more violent suffering you have.
Well, what decide a soul to move to a higher realm or to a lower realm after the current life is over? It’s not decided by Buddha. It’s decided by karma, which is the cause and effect system. Karma can be divided into good karma, evil karma and unrecordable karma. Good karma will lead one to higher realms, and evil karma will lead to lower realms.
Then you might think, it’s easy to understand, that’s why Buddhism encourage people to raise good behavior, because we can move to higher realms and have more enjoyment?
No, that’s completely a misunderstanding of Buddha’s Dharma. Buddha is actually far more merciful than what you can expect. Even one becomes a being in the heaven realm, that’s not forever. Some million or billion years later, when the effect of good karma is consumed up, the evil karma will become prominent. After the death of that heavenly life, the soul probably fall to the the lower realms directly, I mean hell, animal, ghost. Would you like to accept that?