Interesting post. I usually try to stay out of the religious discussion on slashdot, because, really, it's full of people who have heard some arguments against Bible and think they are experts on the subject even without having ever read it. Invariably none of them have heard of the rather clear distinction between moral law (that is supposed to be eternal) and the ritual law that forbade the Israeli from e.g. mixing separate materials in clothing in some ways, to remind them of their covenant with God - which the New Testament has made abundantly clear is not God's current covenant with people (there's a reason it's called the Old Covenant). But no, bashing the Bible with poor reasoning appears to be so fun that anyone should do it, regardless of whether they've ever read it. Come on, people, you can find better arguments against the Bible, they certainly do exist, and you can figure them out. Of course that might require, heavens forbid, actually researching stuff instead of just blindly bashing and repeating arguments invented by equally ignorant people.
But I'm not blaming you for all that. I get the feeling your post contains kind of authentic puzzlement about the behavior of us Christians. That's good. I think it deserves an answer. I'll do my best to explain in the hope that you find some kind of answer to your puzzlement in my words.
What you are wondering about is one quality that I believe truly sets Christianity apart from other monotheistic religions. What you describe is, I believe, actually quite close to Islam (and I'd say that Islam and Judaism are much closer to each other than either to Christianity). Islam teaches us thus, simplifying things a bit: If you can believe in God (Allah), that's undoubtedly good; but even if you can't, the most important thing is that you submit. The literal translation of the word "islam" is "submission" (and "muslim" = "one who submits"). This is reflected in popular muslim names like Abdullah, "slave of Allah".
The relationship of God to people, as reflected in the Old Testament, is somewhat like that, but my feeling is that it's not that strict in a sense. The emphasis, in any case, is on good deeds, and to an extent obeyance. Somewhat in contrast to Islam, the Old Testament still talks about God adopting Israel as its people, and giving them his good law because he knows, like a father setting limits to his daughter, that it's good to the people. Even in the OT, it's more of a parent-child relationship than master-slave. In the Old Testament, the child is the people of Israel.
In the New Testament, the death of Jesus changes things materially. It begins a New Covenant, replacing the old one that the people of Israel had broken so many times anyway. The death of Jesus provides a complete atonement of the sin of mankind. The Bible literally promises that to those who believe, all their sins are forgiven because of the blood of Jesus. In fact, this is a core tenet of Christianity: That every person, no matter how Christian, is a sinner, and repeatedly commits sin; that nothing they do themselves can redeem themselves. However the Bible also promises that we can leave all our sins to Jesus, and he will wash them away. It's not a matter of ego, as you say:
Either these people don't really believe, or they believe but they think their god is lying about the whole eternity bit, or their own ego is so great that they just think god'll forgive them all their transgressions
Indeed, it's hard to believe that God forgives all our transgressions, but that's what the Bible says about the matter. It also says that if the death of Jesus did not atone some of our sins, there would be no way to heaven for any of us. Yes, to a Christian life is a journey towards life less controlled by sin, but nobody will become so good that he would not need God to wash away his sins. This may seem like excusing doing anything at all, because we know we will be forgiven. That's half true - we know we will be forgiven, no matter whether our transgression is a hurting word said to a brother or a whole genocide.
That's where we get back to slavery and submission, but in a perhaps unexpected way. Remember, God's law is good for us humans, but we are incapable of following it because we are tarnished by sin. The NT turns the slavery thing on its head. We, the entire humanity, are born in sin, live in sin and die in sin. We're slaves to sin. We do bad things to others, ourselves and the nature even if we agree they are bad. There's not much we can do about it. Sure, we can strive to live better, getting rid of individual vices, but we're so rotten by sin that no matter how much dirt we peel off ourselves, we're still tarnished, slaves to sin. And because God is infinitely holy, i.e. cannot tolerate sin, anyone who sins is banished forever from the presence of God; there is no heaven for anyone who is slave to sin.
What the New Testament offers us is freedom from that slavery. In it God has offered to adopt us as his own children and heirs. He knows that we will still commit sin until we die, but he has offered to wash us clean and let us start anew every time we come to him. He has promised to love us and care for us like a father cares of his own children. The offered freedom is this: We no longer need to sin. We still stumble, because that's our nature, but the children of God know of something better, not found in themselves (because we're all rotten), but in God, who is fully free of any sin. We find ourselves doing what we know is wrong, but we know that we don't need to be slaves to that.
God's holiness is one of the often forgotten aspects of God amongst those who love to bash. God is infinitely holy, infinitely loving and infinitely merciful. There's no compromise there. Often people only talk about the love aspect and even then equate it to a Father giving his children everything they want. But uncompromising holiness, I think, is the reason why not everyone can go to Heaven. It's simply not possible for sin to enter heaven. Christ's death was a curious solution that allows uncompromising holiness to coexist with uncompromising love, because of the atoning blood paying for our sins that we could not pay for ourselves. Holiness demands full payment for sins. It's not so much God throwing bad people to Hell as it's that being where people are going, and God offering a way out of it, fully paid with an extreme sacrifice because we have nothing to pay with ourselves, to anyone who accepts the gift.
So, you see, the issue is not that we Christians need to constantly watch our words and deeds in the fear that we will be cast to Hell for them. We know we're forgiven. True, we can get occasionally get lost from the good path because of that freedom, and to most Christians life does seem to come in cycles where we find or at least feel ourselves closer to God at times and closer to this world and sin at others (but it's not feelings that matter - we can trust we're saved because it's promised in the Bible, no matter if we feel we deserve it). For many of us, the perceived unconditional love we get from God eventually drives us to try to live better, in the hope that we can love because we're loved ourselves.