In this parable, Jesus had just told the story of a servant who owed his master 10,000 bags of gold and could not repay the debt. The only solution to get the money back was to sell the servant and his family into lifelong slavery. But when the servant begged for mercy, his master forgave his debt (accepted a personal loss and discharged the servant from any further obligation to pay). It was a picture of God's compassion: He has mercy on anyone who asks for mercy, giving them the chance to start again without the debt of sin hanging over them.
 
But as Jesus continued the story, the character of the servant became more obvious. As soon as he left the master, he found another servant who owed him some money (quite a large sum, but tiny in comparison with the amount he had owed his master, and been forgiven) and physically attacked him, demanding his money back. This other servant also begged for mercy and time to pay, but he would not listen. Instead he was sent to prison.
 
This raised an outcry among the other servants. It was not right for the first servant to have been completely forgiven such a large debt and then for him to refuse to forgive the much smaller debt. It was unjust. The first servant had taken advantage of the kindness of the master and had not been grateful for the mercy and grace shown to him. While the master's heart was full of compassion, the servant was full of self-interest and violent greed. He had learned nothing.
 
God's mercy is so wide and His grace so deep that He can cover every sin and meet every need. He forgives so that relationships might be restored, not just to help people escape from punishment or to encourage their greed. The other servants were right to tell their master and leave the matter with him. In the same way, we must bring every injustice to the Lord in prayer, trusting Him to deal with the offenders; it would be unsafe for their fate to be in our hands. Unforgiveness is appallingly ugly especially in those who have been forgiven so much: it reeks of ingratitude and lack of desire for relationship with God.