Sin is considered a transgression against God, who created mankind to be holy and faithful unto Him. Sin is what condemned mankind to death, both physical and spiritual. God worked to redeem mankind from the power of sin by sending His Son Jesus to die for our sins by shedding His blood on the cross, and to rise from the dead three days later.
In the Left Behind series, God uses His judgments throughout the Tribulation to get man to turn away from sin and receive salvation in Him through His Son Jesus before His Second Coming. While many do turn away from sin and become Christians during that time, many more reject the gospel message and instead turn to worshiping Nicolae Carpathia as their god and lord while they continue to practice their sins.
In the Millennial Kingdom, the naturals who reject Jesus as their Lord and Savior instead choose to align themselves with Satan and The Other Light while they live sinful lives. As a result, they go to hell before they are resurrected to face judgment at the Great White Throne and then be cast into the Lake of Fire. Naturals that commit a sin worthy of death could also be terminated before their 100th birthday, as an Other Light member found out when he tried to rape a female glorified in an attempt to impregnate her with a child that would be able to live past 100 years of age without needing to choose Jesus as his Lord and Savior.