Rashi explains how these lowly slaves harmed the Jews. He says, “They, too, subjugated [the Israelites] and were happy at their suffering.”
There is an astounding lesson here. They were being punished for enslaving the Jews; that makes sense. But who cares if they were happy that they were suffering? That was how they felt inside and doesn’t hurt anyone. Why should they be punished for that?
The answer is that failing to be sympathetic to someone is itself a crime. Especially these people who were downtrodden themselves, should have felt and shared the pain of the Jewish slaves. Their callousness was as much a blow to the enslaved Hebrews as the crack of a whip and they should have known better.
We should know better and that’s why the Torah tells us: Don’t be happy at the suffering of another – it may become yours as well.
