We are up to Pirkei Avos, chapter 2, mishnah 7 (and according to some divisions of the mishnayos: chapter 2, mishnah 6), in which we once again encounter the words of Hillel. The mishnah is simple in its wording but deep in its content: it reflects the fundamental principle of "middah k'neged middah" - that the Holy One, Blessed is He, punishes a person in a way that mirrors his own deeds. It also teaches that a chain of violence inevitably produces a chain of retribution, and a person must take this to heart in every area of his life: no evil deed goes unpunished.
The words of the mishnah:
"Af hu ra'ah gulgoles achas shetzafah al p'nei hamayim" - once Hillel saw a human skull floating on the surface of the water. That man had been murdered and his head cut off, and now his head was floating on the river. Hillel recognized the man, who was known as a criminal and a murderer.
"Amar lah: al d'atift atifuch" - Hillel turns to the skull and says: because you drowned others, you yourself were drowned. Measure for measure.
"V'sof metifayich yetufun" - and in the end, those who killed you will themselves be killed.
Hillel speaks here in Aramaic. The plain meaning of the word "atift" is an expression of floating, and in this context it refers to drowning and killing - the floating head testifies to what its owner's own hands had done.
Two points in Hillel's words:
The reason for the victim's death: that man met his death before his time in a manner of measure for measure. Because he was a murderer, he was killed.
The responsibility of the killer: even though it is Avraham's head that is floating on the river, and Avraham did indeed kill Shimon and therefore deserved to die - who killed him? Levi killed him. Avraham's death was justified in terms of his own deeds, but Levi, who has now become the one carrying it out, has himself become a murderer. The fact that Avraham deserved to die does not qualify Levi to be the one who kills him, and the cycle of violence keeps rolling on. In the end Levi will receive his punishment, whether in this world or in the World to Come, and he too will die for having done this terrible deed.
In summary: this mishnah is short and sweet, but a profound principle is contained within it: every action has consequences. No good deed goes unrewarded, and no evil deed goes unpunished. In the words of Rabbi Berel Wein: "The wheels of history grind slowly" - it may take a very long time, but they grind exceedingly fine. The justice of the Holy One, Blessed is He, is precise to a hairsbreadth, and every person ultimately receives what is coming to him. With this Hillel warns a person: no one gets away clean, and therefore it is worth keeping this in mind in every decision of life.