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 (Matthew 18:23-27).  The master could get some of the money back by selling the servant and his family into lifelong slavery.  But when the servant begged for mercy, his master forgave his debt (accepting a personal loss and discharging the servant from any further obligation to pay).  It was a picture of God's compassion (Psalm 86:15).  He has mercy on anyone who asks for mercy (Job 9:15), 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 obvious.  As soon as he left his master, without any debt, 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); he then physically attacked the other servant, demanding his money back.  That other servant also begged for mercy, and time to pay, but his pleas fell on deaf ears.  Instead, he was sent to prison.
 
That raised an outcry among the other servants.  Surely, 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.  It had become clear that the first servant had taken advantage of the kindness of his master, and he was not grateful for the mercy and grace shown to him.  While the master's heart was full of compassion, the servant’s heart was still full of self-interest and violent greed.  He had learned nothing from his master’s personal kindness to him and his family.
 
Jesus told this story to teach His trainee apostles that they needed a new heart which was willing to forgive others (Matthew 18:35).  God's mercy is so wide and His grace so deep.  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 exposes a heart that has no hunger for a restored relationship with God.