Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee?

People's New Testament

18:32,33 O thou wicked servant. The sin of which the servant was guilty and charged is not that needing mercy he refused to show it, but that having received mercy he remained unmerciful still.

Matthew Henry's Concise Commentary

18:21-35 Though we live wholly on mercy and forgiveness, we are backward to forgive the offences of our brethren. This parable shows how much provocation God has from his family on earth, and how untoward his servants are. There are three things in the parable: 1. The master's wonderful clemency. The debt of sin is so great, that we are not able to pay it. See here what every sin deserves; this is the wages of sin, to be sold as a slave. It is the folly of many who are under strong convictions of their sins, to fancy they can make God satisfaction for the wrong they have done him. 2. The servant's unreasonable severity toward his fellow-servant, notwithstanding his lord's clemency toward him. Not that we may make light of wronging our neighbour, for that is also a sin against God; but we should not aggravate our neighbour's wronging us, nor study revenge. Let our complaints, both of the wickedness of the wicked, and of the afflictions of the afflicted, be brought to God, and left with him. 3. The master reproved his servant's cruelty. The greatness of sin magnifies the riches of pardoning mercy; and the comfortable sense of pardoning mercy, does much to dispose our hearts to forgive our brethren. We are not to suppose that God actually forgives men, and afterwards reckons their guilt to them to condemn them; but this latter part of the parable shows the false conclusions many draw as to their sins being pardoned, though their after-conduct shows that they never entered into the spirit, or experienced the sanctifying grace of the gospel. We do not forgive our offending brother aright, if we do not forgive from the heart. Yet this is not enough; we must seek the welfare even of those who offend us. How justly will those be condemned, who, though they bear the Christian name, persist in unmerciful treatment of their brethren! The humbled sinner relies only on free, abounding mercy, through the ransom of the death of Christ. Let us seek more and more for the renewing grace of God, to teach us to forgive others as we hope for forgiveness from him.