The Skull on the Water
He as well saw a skull floating on the surface of the water and he said to it: Because you drowned others they drowned you; and those that drowned you will in the end be drowned. - Avos: 2, 6
Rashi – after commenting on this mishna – says that not all texts include it. Nowadays, however, this mishna is included in standard texts. Rashi doesn't dwell on whether or why it should or shouldn't be included as part of Pirkei Avos but merely notes that it isn't printed in all texts. Rashi didn't publish his commentary inside the gemora. His commentaries were published as stand-alones, without the original text they explained. This is actually the reason that the Tossafos refer to Rashi as "Peirush Hakuntris"—“the explanation from the notebook”. Since not all texts contained this mishna and Rashi nonetheless explained it, he needed to specify that it may not appear in a given text.
All of this still leaves us with the question of why did some texts not incorporate this mishna.
Mishnayos – unlike Gemora – are elemental by nature. They serve as the substratum of halacha. As Pirkei Avos is the tractate that deals with Jewish ethics, it can only include precepts that address each and every Jew in a direct manner. The above cited vignette thus should not be included as it seems to be merely an anecdote rather than an axiom that can be adopted by all. Why then is it interpolated in some texts?
In order to answer the question we need to understand what Hillel was communicating in this anecdote. Hillel sees a skull afloat in the water and he says unto it: because you drowned others you were drowned and likewise those who drowned you will be drowned as well. How did Hillel know that the fellow whose skull was floating in the water drowned anyone else, and how did he know that those who drowned him will themselves be drowned? Was this some sort of prophetic knowledge that Hillel possessed or was it some sort of perception through acute acumen? Moreover, who was Hillel talking to – to the dead man's skull?
How Hillel knew or understood this whole story is in truth irrelevant. Hillel certainly wasn't talking to a dead man's skull. Hillel was talking to us and he used the floating skull as the premise for what he wanted to relay. Society is made up of individuals. While it can include multitudes of individuals it will always remain a community of individuals. Consequently, social decorum will always be affected by an individual's acts. Hillel is thus telling us that when one kills someone, the killer has forced society to become cognizant of murder as a reality. Recognizing murder as a reality means that it has consciously been accepted by society. Once murder has become accepted by society it is likely to reappear as a social norm. Kayin after killing Hevel was afraid of this, because that's how society works. Each time an act of corruption has occurred, that act has warped the fabric of social decorum. The more such acts continue to be perpetrated – the more they become not only part of the social decorum – they become a social norm. Hillel started by saying because you killed you were killed, but he finishes off by saying those who killed you will be killed. Hillel has shifted from the singular to the plural. This is because an individual act leads to another individual act, but once there are multiple individual acts a transformation creating a warped social norm has occurred.
Our personal actions shape society. We cannot escape this because we are all part of Am Yisroel, and we are all part of the global community. Unfortunately, evil deeds ultimately create a horrific society, but fortunately good deeds create proper social decorum. It is in our hands to create and fashion a better world. Each one of our acts of kindness and goodness forges positive social behaviors. We can create a better tomorrow.
It is thus that while this mishna appears to be a random anecdote it is in truth – an invaluable teaching of Hillel – worthy of being included in Meseches Avos.
Hashem gave us the Torah and has laid out for us definitively good and bad, right and wrong. It is our job to create an ethical community. When we do so we will be able to serve as a light onto the nations and as a mamleches kohanim – a priestly nation. It is then that Hashem will be able to use us as a chariot to bring His goodness and glory to the entire world.