Avot 2:7 He [Hillel] 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.”
He [Hillel] also saw a skull floating on the water;
The word ‘also’ appears here because in the expanded version of Pirkei Avot by Rabbi Nathan,  this is the second incident experienced by Hillel. The first he describes was an encounter with two sets of merchants who were selling wheat at different prices. When Hillel queried the second group as to why their prices were higher, they explained that it had cost them far greater effort and expenditure to bring the wheat to the market.
As a customer he saw the same product but did not know the background details. In comparison, we might see the same goals achieved in the lives of people and the similar results of their labors, but only God knows what was expended for each one to reach that goal and, hence, how they will be rewarded.
…he said to it: “Because you drowned others, they drowned you; and those who drowned you will be drowned eventually.”
Next, then, Hillel was walking along the bank of a river and saw a skull floating in the water. As the body was decapitated and not given a proper burial, we can assume he was the victim of foul play. Hillel addresses the skull as if he is an advocate in a court of justice and explains a principle of Divine retribution: “If you do harm to others, harm will be done to you, but eventually those who harm you will also be harmed.”
This is an echo of one of Hillel’s most well-known sayings. When he was challenged to sum up all of Torah in a nutshell he said, in effect, “What you do not want others to do to you, do not do unto them.” In Yeshua’s signal Sermon on the Mount he teaches a positive form of the same principle, “As you wish that others would do to you, do so to them” (Lk. 6:31).
Midrash records how the renowned Rabbi Akiva once was journeying between towns with one of his students. As they walked they saw a roadside well in the distance with a weary traveler asleep alongside it. They then noticed another traveler approach the sleeping man, rob him of his money bag and run off in the direction of the next town. They hurried to reach the victim and woke him with the bad news. The man was distraught and he explained that the money was the inheritance his father had left for him and it was all he had in the world. When they proceeded on their way, the student bitterly expressed how unfair life was and questioned how God could allow such injustice. Rabbi Akiva stopped and with prophetic insight advised his student that he would only speak to him on the subject of Divine justice this once and never again.
He told him that the robbed man’s father had gained the money in the bag by unfair means when cheating a poor farmer. The father of the robber himself had been a murderer. When the robber reached the next town, he turned down an alleyway where he was recognized by the son of the murdered man, who promptly attacked and killed him in revenge. In the struggle, however, the money bag dropped to the ground. The assailant dragged the body of the robber and dumped it in the nearby river. A while later, a man happened upon the bag of money in the alley. He reported it to the courts and when an owner was not found he was told he could keep it. Who was this man? The son of the poor farmer! This story makes one’s head spin somewhat and clearly illustrates the modern expression, “What goes around comes around!”
Although it cannot fully be understood in this world, belief in Divine justice is an essential principle of biblical faith. It is one that requires acceptance by faith, as in most cases it is difficult to understand from a human perspective. So many questions can be raised such as, “Why, Lord? Why did the child have to die? Why did You allow this illness, that accident, that tragedy, that unfair persecution?” The range is limitless and can be summed up in the now classic question, “Why do bad things happen to good people?” Obversely, we also can consider the question, “Why do good things happen to bad people?” Often, those who are blatantly sinful, who either ignore or mock God and His Word, seem to prosper and gain fame and fortune. Why?
Moses met face to face with God and learned directly from Him. When he was troubled by the suffering of innocent people and asked God for an explanation, he was told that it was something the human mind could not grasp in this world. He would need to wait until his soul left his body and was raised to the Presence of God in the heavenly courts before he could understand Divine justice (Talmud – Berachot 7a). 
Only God can see the eternal and infinite picture of both the world and an individual life. We see only a very small piece of the vast, unending puzzle of life. Our trust is secured in the fact that we know, according to His eternal Word, that the rewards and punishments of life are ultimately fair and for the highest good of those who love Him. We read in the book of Hebrews, “Without faith it is impossible to please him, for whoever would draw near to God must believe that He exists and that He rewards those who seek Him” (11:6).
A beautiful promise of reward is made in the book of Ruth: “The Lord repay you for what you have done, and a full reward be given you by the Lord, the God of Israel, under whose wings you have come to take refuge!” (2:12).
In the love of Messiah, from which nothing can separate us, we know “…that for those who love God all things work together for good, for those who are called according to His purpose” (Rom.8:28). Nothing happens without a reason in God’s scheme of things and, when our lives are entrusted to our faithful Father in heaven, we always can say with confidence, “Gam zu le’tovah!” “Even this is for the good!”
1. Avot De-Rabbi Natan
2. Rabbi A. Twerski, Visions of the Fathers; 102