There's a formidable sense in which death is a curse. It's a curse for the damned. And it's a curse for survivors who lose loved ones.
Yet there's a sense in which death can be a mercy. A fallen world is a minefield. So many things can go terribly wrong. Suppose humans were immortal in a fallen world. Suppose humans were youthful, ageless, and healthy.
But that could just be a different kind of hell on earth. If you live long enough, your luck will run out. If you live long enough, you will step on a land mine. The longer you live, the more heartache you'll experience.
At best, life in a fallen world will be tedious. Imagine the unbearable tedium of immortal life in a fallen world. It's my impression that some people stay married because they get to a point in life where it's too late to start over again. It's not because the couple is so devoted to each other. It's that they have too much to lose by divorce and remarriage. But if sinners were immortal, how many marriages would last for the duration?
Likewise, how many friendships would end in betrayal or boredom? By the same token, how long could your Christian faith hold out? Wouldn't the Bible become interminably familiar? The sameness would become deadening.
Natural immorality doesn't make you indestructible. Just that you can't die from old age or disease. But you could still be killed. Or horribly maimed or disabled. And you'd stay in that condition indefinitely.
If immortality means the body has greater regenerative resources, evil men could torture you, wait for you to recover, then renew the torture ad infinitum. Yank your teeth out with pliers every few months.
One advantage of death in a fallen world, especially for Christians, is that suffering, now matter how horrendous or depressing, will come to an end. There's only so many times the worst thing can happen to you. When you die, you put all that behind you. Nothing worse can happen to you. Nothing bad can happen to you anymore. The worst is behind you, and you only have happiness to look forward to from hereon out.
And however bad things are, life is short. Like a prisoner, you can mark off the remaining days on the calendar. (I'm referring to Christians. For the damned, it's just the opposite.)