Simply because the mortician tells them only new life can defeat death, they all become convinced that letting Isabella have her baby will save all of them.

According to one of them, Death will be forced to give them all a clean slate. This makes no sense. They're literally just pulling it out of thin air. Since they didn't have anything to do with the baby living and Kim cheating death by being saved is what really caused it makes more sense

I understand that they thought Isabella shouldn't have been alive to give birth, but that doesn't mean that death would give ALL of them a clean slate. At least in part 5, their logic makes a bit more sense of if they kill a person, you can take that person's remaining time, and they're taking your place, balancing things out, but even that's just guessing.