“So also will my heavenly Father do to you, if you forgive not every one his brother from your hearts.”
The story ends with an application. This is how His Heavenly Father will behave (not the torturing but the calling to account) towards all who do not forgive their brother or sister from the hearts. The reference to brother indicates that primarily this applies to forgiveness between ‘brethren' within the circumstances laid done in Matthew 18:15-20. But we cannot limit it to that, for the idea is that having been forgiven we will be living our lives in the light of Chapter s 5-7, in terms of the Sermon on the Mount (and especially Matthew 6:14-15). By it Jesus is saying, ‘freely you have received, freely give'.
It should of course be noticed that the king offered his forgiveness first before the servant was expected to forgive. It was not that the servant had to earn forgiveness. His crime was that having himself been offered full forgiveness he refused a lesser forgiveness to another. His lord had given him the example, that he might follow in his steps. And the point behind it is that he had no real consciousness of the forgiveness that he had been offered, for had he really been conscious of it, it could not have failed to stir him to forgiveness of a fellow servant. (All his fellow-servants saw that). For us it is a reminder that if we have been truly forgiven, and are conscious of it, then it cannot have failed to change our lives. And if it has not done so we need to ask ourselves whether we really have received forgiveness. For the consequence of our forgiveness is that we are to be perfect even as our Father in Heaven is perfect (Matthew 5:48).