In The Problem of Pain, C.S. Lewis says
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.
Is there a specific example that Lewis was likely to have in mind? What is the “very natural process” by which pity for oppressed people leads to a reign of terror?