You think that only Christians will go to heaven?
I do believe that only those that have been saved will go to heaven with the exception of children too young to understand and those people who cannot understand because of learning disabilities.
Do you think that any diabolically evil souls exist? That is, do you believe that in the history of humanity anyone has done anything in which they truly felt was purely evil and in no way were trying to rationalize (even to themselves) their actions?
I cannot conceive of such a being; I have a harder time conceiving of a diabolically evil being than I do of a God. There are a lot of individuals who commit evil in the pursuit of pleasure; but, those individuals are somehow justifying that pleasure is good and ought to be sought. Hitler, Mao, Stalin, Pol Pot...each of these brutal dictators had incredibly detailed justifications for why they truly felt their actions were good and right.
Diabolical evil means that individuals do not happen to commit sin because they are weak willed and God gave them free will; diabolical evil means that, if there is an omnipotent God, God created pure evil.