That's what's cruel. God as he is described in the Judeo-Christian religions is a tyrant. If his "ultimate good" is good just because he says so, it's just as arbitrary as my declaring what's good and what's bad.
He has no legitimate claim to authority.
Don't mistake the story Domination3000 tells as the belief of "the Judeo-Christian religions". It's just one strand within them that conceives of God in this way.
That was why Christ died.
Basically it works like this:
Man sinned. The punishment for sin is eternal separation from God. I am not God, I cannot tell you why God chose that punishment, but I'll quote the common answer anyway and say because God can't allow sin in his presence.
God sends Christ his Son to die on the cross to pay for that sin. He then offers us the free gift to take it and go to heaven. If we accept it, we go to heaven, if not, we die in our sins by our choice and go to eternal separation from God in Hell.
None of that addresses what I said before. Moreover, it's riddled with holes in its own right, as many Christians have been saying for centuries.
First, the doctrine you have just outlined is known as "penal substitution". It is just one version of the Christian doctrine that salvation comes through Christ. In fact, it originated in the Middle Ages; it can be found in some writings of Peter Abelard (ironically, since Abelard is normally associated with a completely different understanding of the nature and mechanics of salvation). Before that time, most Christians believed a completely different story (one originating in the work of Gregory of Nyssa). After that time, the doctrine of Anselm of Canterbury was dominant in the west, which had similarities to "penal substitution", although it was different.
Only at the time of the Reformation did "penal substitution" become widely believed, through the works of Luther and Calvin, and it's debatable to what extent either of them can be considered a proponent of the doctrine. At any rate, it became closely associated with Calvinism and from there entered into evangelicalism, which is why it's believed by most evangelicals today. However, it is a minority doctrine within Christianity as a whole, considered throughout history.
As to the flaws with the doctrine itself, they are obvious. If the punishment for rejecting God is "eternal separation from God", and Christ suffered everyone's punishment on their behalf, then he should have suffered eternal separation from God. But he didn't; at most he was separated from God for a limited period, because he returned to union with God. Moreover, if the punishment for
one person rejecting God is eternal separation from God, then if Christ suffered the punishment due to a single person, that is what he should have suffered. But we are told that Christ suffered the punishment due to many people, perhaps the punishment due to all people. In that case, he must have suffered eternal separation from God multiplied by the number of people who are saved. But how can you multiply
eternal separation from God by any number? How could you distinguish between Christ suffering the punishment due to a single person and him suffering the punishment due to many people? The notion is incoherent; therefore, Christ could not have suffered the punishment due to many people.
Moreover, the notion of one person suffering a punishment on behalf of another is pretty weird. What's the point of it? There are various reasons why we punish people - retribution, rehabilitation, deterrent to others. It is hard to see how any of these are achieved by God punishing Christ in the place of sinners. It's like someone taking a bath on my behalf. It won't make me clean. To return to the "crime and punishment" model, suppose the government proposed to stop locking up criminals, on the basis that a very generous innocent person had offered to go to prison in their place. Would that be a sensible policy?
Moreover, it is inconsistent to say that Christ suffered the punishment, and yet that that punishment can still be meted out to others. If he suffered the punishment, then that should be it. The debt is paid or whatever metaphor you wish to use. If it is indeed just for one person to be punished in the place of another, then it is unjust to punish the other person
as well. It follows that, if you think Christ has suffered my punishment already, it makes no difference what
I choose to do; the punishment has been dealt with, and I will not suffer it. (This is a flaw of all "objective" theories of the atonement: if salvation comes about because of something like this, then it is not dependent upon any decision that I make.)
To put it another way, you can't shift metaphors in the middle of the story like that. First you talk about "punishment". So salvation consists in not being punished. Then you shift and you talk about a "gift" that I may choose to accept or not. So now salvation is a gift that I must actively choose. But these are inconsistent. If someone is
not punishing me then that's the case whether I choose it or not. It's incumbent on the person who believes in "penal substitution" to explain why, if my punishment has already been dealt with, I need to "accept" the state of not being punished, and why it doesn't follow automatically from the fact that my punishment has already happened to someone else.
Considerations such as these are why, although most evangelical Christians believe this doctrine, most theologians do not, and prefer alternative understandings of salvation. Personally I think that the biblical doctrine offered by Paul in Romans chapter 6 is superior to any, and certainly far superior to this "punishment" model.