I believe in afterlife, and I hope it exists. Why? I think there is a need deep down me to be eternal. Do you believe that your soul which is your part that enjoys learning, memories, love and all other spiritual experiences dies?
I hope there is a heaven and hell because there is another need inside me for justice. Frankly I don't think a person who kills one soul is the same as those who had gotten thousands of people killed in a war.
But the good thing about heaven and hell is that we don't decide who goes where, that decision is God's. I think God would give opportunities to those who deserve it to ask for redemption but those dark souls that have forgotten him wouldn't be given the chance to remember him and therefore their souls would remain in the dark. It is probably our worst punishment and fear to be forgotten by him.