The whole human race, past, present, and future is already on the way to hell. How to escape death is really the issue, not what we believe about the end, hell, or our mistaken ideas of how that final judgment scene might look. If there is no escape available it hardly matters that we think an endless hell is cruel. The only thing that matters is that this very day might be my last. Sin is in the world because death is in the world.
Death is in the world because sin causes it.
Paul is referring to the sin and death of the first human and all of his descendants. Adam sinned and he and all of creation, both in the sky above and on Earth, died. The whole cosmos started falling down that elevator shaft. Your question left out the fact that we were all born damned. Whether or not one believes that there is a God, judgment, angels, devils or hell doesn't have much to do with it.