John Gill Bible Commentary

Shouldest not thou also have had compassion..... It is but reasonable, what ought to be, and may be expected, that such who have received mercy, should show mercy; and as the Lord had compassion on this man, and had forgiven him such an immense sum, and saved him, his wife and children, from being sold for bondslaves, the least he could have done after this, would have been to have followed such an example, and have had mercy, as his Lord says to him, on thy fellow servant; between whom, and him, there was not so great a distance, as between him, and his Lord; and the sum so small that was owing to him, as not to be mentioned with his: even as, I had pity on thee; such an instance of pity and compassion did not only set him an example, worthy of his imitation, but laid him under an obligation to have acted such a part.