Matthew 18:30 - Ellicott's Commentary On The Whole Bible

Till he should pay the debt. — Neither the memory of his lord’s mercy, nor any touch of pity, restrains the man who broods over the memory of wrong. But the course which he takes is, it may be noted, as unwise as it is ungenerous. He, as a slave, cannot command his fellow-slave to be sold. He can cast him into prison; but in so doing he cuts the debtor off from all opportunities of gaining the money by which he might pay his debt. His vindictiveness is so far suicidal. This surely is not without its analogue in the interpretation of the parable. Whatever be the nature of the offence, patience and forbearance at once encourage and enable the offender to make restitution. Harshness shuts him up as in the prison of a sullen defiance.