Hell didn't really come into my thinking much when I became an atheist - most of the scripture teachers I had focused on the role of God on earth and in our lives - but it has come to bother me over the years. I can understand, to an extent, that perhaps a belief in hell follows from certain Christian doctrines, or that perhaps that it might be a good motivator to help people do good. After all, there are similar analogues in other systems of thought, so it's not hard to imagine that the idea of hell is such an implausible notion in itself. But thinking about it, what bothers me is that people aren't horrified by that notion, and that some even relish in the prospect.
To forget all the problems with reconciling a "good" God with hell - the inconsistencies are pseudoproblems in my view - the concern is how such a belief has an effect in the real world. The only reaction that one could adequately have to such a notion is to try to do the utmost they can to save as many people from the fate as possible. If they aren't, then they haven't adequately comprehended just what it would mean. Of all the human analogues to hell, of torture and torment, of forced dehumanisation, these are things that as a species we have decided as being unacceptable to us. If anyone were to defend the likes of Hitler and Stalin for their crimes against humanity, they would be seen as defending the indefensible. Yet if hell is anything like some of the fantastical visions that makes it so undesirable to begin with, then we're facing up to something infinitely worse! To not try to save everyone possible would mean that there's something wrong with the person in question.
Yet it's not just inaction that's the concern, but the rationalisation that those who are in hell are there because they deserve it. From Answers In Genesis: "Even though God loves us, it is because He is completely good and just that He cannot simply overlook or tolerate evil—He hates it!" In other words, not only can you not be in God's presence because of the sins of some fruit-eating ancestor, but it's only good and just that you will spend eternity in tortured agony for that ancestral transgression.
Following on the Answers In Genesis page: "Every person is a sinner by nature and by choice (Romans 5:12-21; Isaiah 53:6), and we cannot do anything to purify ourselves; no amount of “good” works can cover what we have done wrong. In fact, the Bible says that all of our righteousness is like a pile of “filthy rags” before the holy Creator God (Isaiah 64:6)." So we can't help but be horrible and wicked, and even if we appear to be doing good, it's still nothing. As they say: "This is justice. Everyone stands guilty before God, and if He were to condemn all of humanity, that would be just—because we all deserve punishment of eternal death in hell."
That someone could even write those words should be very disheartening. That many people believe it is downright dangerous. What good can come of a view of humanity where all action except belief in the vicarious atonement of the godman is deserving of an eternity of torture? That everything but accepting Jesus' "sacrifice" warrants an eternal punishment that pales in comparison to what the most evil of evil people on this planet has done. What value could there be in human life if human life is irredeemably wicked? I'm reminded of the saint Zarathustra encounters: "Now I love God: men, I do not love. Man is a thing too imperfect for me. Love to man would be fatal to me."
The hope would be that anyone who believes in such a notion doesn't live by it; that pragmatism guides their actions in this life while their belief of the next world remains otherworldly. Words like good and just have no earthly use when framed in such a manner other than to cast others in a dehumanising light. Even some of the causes taken up by hell-fearing fundamentalists doesn't make much sense in light of what counts as good and just. What good is fighting against abortion or homosexuality when neither of them is any more deserving of hell than giving to charity is? It's stroking one's own sense of righteousness in terms of that which is declared to be sinful!
But even if one takes the AiG interpretation of good and just as being wrong, and that hell is not a matter of heresy but one of action, the thought of a punishment amounting to eternal torture is still just as problematic - just that such a punishment sits better with our moral intuitions. It would not be unreasonable to think of Hitler being punished, but even Hitler being tortured cruelly and endlessly should be something that even the most vengeful of us should be horrified by.