A few quick thoughts, without full scripture quotes:
1. Throughout scripture, old and new testament, you have a consistent picture of a Holy God who has perfect wrath and justice toward sin. He has promised to punish sin. If he did not have wrath toward sin, then he could not be called a truly GOOD God. "What good judge lets the guilty go free."
2. On the cross, Jesus took our sin on himself. Romans says that While we were God's enemies, Christ died for us. Jesus paid it all. Romans also says that by faith, we receive his perfect righteousness that he purchased for us.
3. In Hebrews 12, we get the fullest description of God's dealing with his children. For the Christian, discipline is not for punishment (retribution of wrongs done), but for training in righteousness as a father disciplines his children. Jesus already took all the punishment I deserve for any sin, past, present, future. So the discipline that now comes is not for punishment, but for training, guidance...for my good.
***my favorite illustration is this: Suppose I (as a Christian) stop giving to my church offering because I want to buy big new TV. SO I do it, and immediately God allows my roof to cave in, crushing the TV, and half my house. Is this God punishing me because He's a holy God and has to punish sin, and because I deserve it.? NO! I know this because what I really deserve for that sin is not a crushed TV, but eternity in hell. God has not given me what I deserve, but has in fact been merciful to me by giving me something to teach me a lesson about my behavior with money. The roof collapsed for my good.***
So to summarize: Will not the Judge of earth do rightly?
-God is interested in giving Justice in punishment to those who continue in their sin in rejection of Him.
-God is interested in giving both discipline (for training) and rewards to his children, who he sees as completely forgiven and needing no further punishment because of Christ.