Charles Ellicott Commentary Matthew 18:30

"And he would not: but went and cast him into prison, till he should pay that which was due." — Matthew 18:30 (ASV)

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 the wrong. The course he takes is, it may be noted, as unwise as it is ungenerous. As a slave, he cannot command his fellow slave to be sold. He can throw him into prison, but in doing so, he cuts the debtor off from any opportunity to earn the money to pay his debt. His vindictiveness is, in this way, self-defeating.

Surely, this is not without its parallel in the interpretation of the parable. Whatever the nature of the offense, patience and forbearance both encourage and enable the offender to make restitution. Harshness, by contrast, shuts him up as if in a prison of sullen defiance.