We cannot on one hand praise the King for forgiveness of a monstrous debt and then have him wink at the unjust way an offender turns on the one who owes him a small amount.
For the King’s mercy to mean anything, there must be the threat of justice and punishment to the one who recklessly offends the King in unthinkable ways.
If mercy is the atmosphere of Heaven, then justice is the atmosphere of Hell. The mercy of God is freely offered through Jesus’ sacrifice to the worst offenders. It is mercy beyond what we deserve or can earn. The gospel is good news in affirming that all sinners can be forgiven, and have a new Jesus-like nature develop in them.
The mercy of God defines His Kingdom. Jesus said the merciful are shown mercy. Conversely, the unmerciful will not be shown mercy. People who reject the notion of a Biblical Hell want the mercy of God without God’s right to judge between good and evil. If evil is really just a lower form of good, then all must eventually become good and we do not really need justice.
Jesus will have no such lack of justice. The King of His story sends the man to the torturer’s prison where the debt can be atoned for. The impossible size of the debt means the man will be there forever.
The King was right in the first place to require justice against a true offender. The mercy was an undeserved gift that only had meaning if a real punishment were averted. Jesus portrays a very merciful King who will revert to justice when His mercy does not make a lasting effect on the recipient.
It is no small sin to beg for mercy from God and still demand just punishment from all who need your mercy. This is Jesus’ point. Unless you can forgive others from your heart, you did not understand the mercy God showed you.
Those who know God’s mercy must operate on the principle of mercy. If they do not show mercy but insist on justice, they will not receive mercy, but justice. An unforgiving heart is an unforgiven heart and is subject to torment “until he should pay all”. A truly forgiving heart is one result of spiritual rebirth.