Pirkei Avot 2:7

"He also saw a skull floating on the water, he said to it: 'Because you drowned others, they drowned you; and those who drowned you will be drowned eventually.'"

This perek is as difficult as it is short.  The apparent message is that G-d makes perfect justice in the world.  Hillel states without knowledge of the circumstances that the dead person whose skull floats on the water drowned others, and was drowned as punishment for the offense, and those who drowned him will be drowned eventually.  This is very jarring.  Surely, millions of people died, sometimes at the hands of others, who did not "drown" others.  It is perhaps easier to accept the possibility that people who "drown" others will be drowned eventually.  But what of the victims of the Holocaust?  The babies who were thrown into the fires or went into the gas chambers in their mothers arms?  Who did they "drown"?

The only way to understand this is not as a universal rule, but rather an observation about injustice on earth.  Hillel is saying that ruthlessness creates enemies, who will exact their revenge, and that this imperfect human reaction is an instrument of Divine justice.  Mussolini and Hitler made enemies through their ruthlessness, who worked for their undoing.  Lesser malefactors also make enemies by their harmful acts, who lead to their undoing.  As in Chad Gadya and the course of Jewish history, the slaughters and the slave drivers are brought low, and the Jewish people, with G-d's help, survive and continue long after the slaughterers and the slave drivers are forgotten.  This does not mean that everyone who dies brought it on himself; in fact, the most holy people die.  But we must be aware that Divine justice does not permit evil to go unpunished in the long run.  Humans are the instruments of His justice.