As you already know, Moshe Rabbeinu grew up in the palace of Pharaoh and one day he has decided to go out and see the world outside of the palace. As he was taking a tour around the city, he saw all the hardships and bitterness his people had to go through in their everyday life. Suddenly he came across this scene - he saw how an Egyptian guard was beating a Jewish slave.
In Shemot 2:12 says:
וַיִּ֤פֶן כֹּה֙ וָכֹ֔ה וַיַּ֖רְא כִּ֣י אֵ֣ין אִ֑ישׁ וַיַּךְ֙ אֶת־הַמִּצְרִ֔י וַֽיִּטְמְנֵ֖הוּ בַּחֽוֹל׃
“He turned this way and that, and seeing no one about, he struck down the Egyptian and hid him in the sand.”
Rabbeinu Bahya said:
וירא כי אין איש אין איש עתיד לעמוד ממנו שיתגייר - And he saw that no one, (there are were people standing around since pharaoh found out about this incident), no one was a man enough to stand up to him - to the Egyptian guard. All were afraid and no one stepped forward to defend the slave.
So Moshe Rabbeinu struck the Egyptian guard, killed him and buried him in the sand.
In the Masechet Gittin 56A there’s a story of Kamtza and Bar Kamtza, where the host of the party was arguing with one of the people that was invited to the party erroneously, and which happened to be his enemy - bar Kamtza. As they were arguing back and forth, no one from the rabbis present at the party said anything in his defense. When he was thrown out from the party, bar Kamtza decided to revenge all those people involved in his humiliation and the outcome was that we have lost millions of lives and the Temple was destroyed.
In another scenario: when Moshe Rabbeinu came late for few hours from the Mount Sinai where he received the Torah, the people came over to Chur the son of Miriam and demanded from him to build them an idol - he flatly refused for which he was killed said Rashi on Shemot 32:5:3. After killing Chur they have approached Aaron haCohen. He saw how they have dealt with Chur and agreed to build them an idol, but he was stretching the time and slowing the process in hopes that Moshe Rabbeinu will come down and this plan will be foiled. But against all of his efforts and with the help of the Egyptian sorcerers working behind his back, the Golden Calf has emerged from the fire and people have worshipped the idol as it says in Shemot 32:4. When Moshe Rabbeinu came down with the commandments, he saw similar scenario, few thousand or so people are dancing around the golden calf and the rest of the three million people along with Aharon haCohen are standing around and watching.
After this incident, Aaron haCohen becomes a Rodef Shalom - the Pursuer of Peace. Aaron haCohen understood one simple truth – we should not live on autopilot. He started strengthening the unity between friends and families, thus creating peace, unity, and harmony in the society.
But what is living on autopilot? When we live on autopilot – we make mistakes in ideology, principles, and convictions. We ignore quarrels in the society, accept and spread slander, and play along in other people's arguments.
Evil thrives when no one goes against it when no one challenges it and it sweeps more people into it while doing lots of damage.
As German theologian Martin Niemöller said in his poem in 1933:
"First they came for the socialists, and I did not speak out—
Because I was not a socialist.
Then they came for the trade unionists, and I did not speak out—
Because I was not a trade unionist.
Then they came for the Jews, and I did not speak out—
Because I was not a Jew.
Then they came for me—and there was no one left to speak for me."
And as Albert Einstein said: “This world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.”
The more we ignore things that are happening with others, standing by idly while letting things to be done and words to be said, the more it goes out of control and at the end, we too become liable in the eyes of G-d. Look around in your community, your family and the places where you have influence. Is it lacking peace? Is there are harmony or there are strife and slander?
Moshe Rabbeinu had to get involved when Egyptian guard was beating a Jewish slave to death and he did. The rabbis that were present at the party, had to get involved in the story of Kamtza and Bar-Kamzta without concerning themselves of the consequences they might have had. And in the building of the golden calf, the three million people and Aaron haCohen had to stand their ground and go against the people that were bowing down to the idol no matter the outcome that was there for them.
The more we keep quiet, thinking that it is none of our business and while pretending we do not notice anything – we give evil time to carry out their evil plans, because that's when they start to count on us that we will defend them, justify their actions before others and not expose their plans – and for this we carry the burden of the guilt and will be responsible before G-d, as our ancestors.
So, what should one do? Should he get involved in a machloket/dispute? Definitely YES! but not to make more trouble or take sides, but to make sure the dispute is ended and peace is restored.
That's why in Pirkey Avot 1:12 it says: "Hillel used to say: Be of the disciples of Aaron haCohen, loving peace and pursuing peace..."
When Aharon haCohen died, on his funeral there are were a lot of children from all those families that were about to fall apart, that he was able to save. Thousands of children were named in his name as a token of appreciation from their parents. Aharon haCohen has dedicated his life to peace, he was pursuing it, he has busied himself with it all the hours of his day – giving it the importance and letting us know of the huge responsibility that was placed on our shoulders.
With his efforts and results, Aharon haCohen has proven that each one of us can and should be a peacemaker in his own society.