They are punished for unbelief, yes, but also for their sins:3. Souls that are condemned are so punsihed not becuase of their sins, since it was already paid for by the death of Christ, but because of their unbelief, which resulted to them lacking the rigtheousness that Christ's blood has for them.
But because of your stubbornness and unrepentant heart you are storing up wrath for yourself in the day of wrath and revelation of the righteous judgment of God, who will render to each man according to his deeds..
God's wrath is meted out to unbelievers for their deeds--for their individual sins.