As Aquinas teaches, Christ did not die to save the fallen angels, because they cannot be saved. They cannot be saved because their wills are locked on to evil. It is impossible for them to repent. It is impossible for them to repent because they are incorporeal, and thus lack the bodily preconditions for the changeability of the will's basic orientation toward either good or evil. An angel makes this basic choice once and for all upon its creation. It is because we are corporeal that Christ can save us. But he can do so only while we are still in the flesh. Upon death, the soul is divorced from the body and thus, like an angel, becomes locked on to a basic orientation toward either good or evil. If it is not saved before death, it cannot be saved. It's game over. I explained the reasons for all this in a post on the metaphysics of damnation.
Now, there is an exact parallel with the condition of the saved. They cannot be unsaved, and for the same reason. Their wills are locked on to good. It is impossible for them, after death, ever to fall away again into evil. You might say that that just is heaven, or what is fundamental to heaven. It is the impossibility of ever doing evil. It involves rewards beyond that, of course, but the rewards follow upon the fact that you are forever safe in only ever willing good, and thus can be forever worthy of such rewards.
The parallel is so exact that you cannot deny hell without denying heaven.
As Aquinas writes:
It was Origen's opinion [Peri Archon i. 6] that every will of the creature can by reason of free-will be inclined to good and evil; with the exception of the soul of Christ on account of the union of the Word. Such a statement deprives angels and saints of true beatitude, because everlasting stability is of the very nature of true beatitude; hence it is termed "life everlasting."
If the wills of the damned could change after death, then so too could the wills of the saved. Thus, they wouldn't truly be saved any more than the former would truly be damned. They would forever be in danger of falling again into evil and facing punishment for doing so. The travails and instability of this life would never end. Hence, no hell, no heaven either.
But doesn't the parallel break down insofar as God could simply annihilate the damned souls while preserving those that are saved? No, and for two reasons. First, as I have argued elsewhere, there is a sense in which the damned perpetually choose to continue existing insofar as their will is locked, upon death, on a certain (evil) way of being, rather than on non-being. God gives everyone what he wants. It's just that what the saved perpetually want is a way of being that is good and what the damned perpetually want is a way of being that is evil. Second, there are consequences to getting what we want. It is often said that we damn ourselves, and that is true. But that is only part of the story, and as I have argued elsewhere, there is also a sense in which God really does damn us. For good and evil choices merit, respectively, rewards and punishments, so that just as someone who perpetually chooses good perpetually merits rewards, so too do those who perpetually will evil perpetually merit punishments. And in both cases, God ensures that this is exactly what they get. Again, the parallel between heaven and hell is exact.
This is just cold, hard metaphysical reality, and has nothing to do with what the defender of the doctrine of hell wants. Suppose there's a fork in the road, the right side of which leads safely home and the left side of which leads to a yawning chasm. Suppose I veer left and you warn me to turn back before I drive off the cliff and meet a fiery end. It would be extremely bizarre if I responded to this friendly advice by accusing you of wanting me to die in such a crash, and insisted that if you really cared about me you would tell me that the left road too leads home, or at least will lead only to a minor and temporary inconvenience (a roadblock, say) rather than to death. The truth, of course, is that you want me not to be harmed and that that is precisely why you are warning me, and that if you were to tell me that a left turn would not lead to a fiery death you would be deceiving me and putting me in grave danger.
But blaming the messenger in this irrational way is precisely the reaction of many critics of the doctrine of hell. They accuse the defender of the doctrine of lacking mercy, and of wanting people to be damned. This is delusional, and as with the motorist who ignores all warnings and keeps speeding toward the cliff, it is a delusion that only increases the danger of calamity. But in neither case can the delusion last. The soul in danger, like the motorist in danger, will, one way or the other, realize eventually that the warnings were accurate. The only question is whether he finds out the easy way or the hard way.