What did C.S. Lewis’s think was pity without moral law leading to terror?

In *The Problem of Pain* Chapter 4 (Human Wickedness), C.S. Lewis says > 6. Perhaps my harping on the word ‘kindness’ has already aroused a protest in some readers’ minds. Are we not really an increasingly cruel age? Perhaps we are: but I think we have become so in the attempt to reduce all virtues to kindness. For Plato rightly taught that virtue is one. You cannot be kind unless you have all the other virtues. If, being cowardly, conceited and slothful, you have never yet done a fellow creature great mischief, that is only because your neighbour’s welfare has not yet happened to conflict with your safety, self-approval, or ease. Every vice leads to cruelty. Even a good emotion, pity, if not controlled by charity and justice, leads through anger to cruelty. Most atrocities are stimulated by accounts of the enemy’s atrocities; and pity for the oppressed classes, when separated from the moral law as a whole, leads by a very natural process to the unremitting brutalities of a reign of terror. Did Lewis ever indicate what examples he was thinking of? What is the “very natural process” by which he thought pity for oppressed people would lead to a reign of terror?