Chur's Death and Aharon at the Golden Calf
Chur stands up to the people at the Golden Calf and is killed. Aharon HaKohen, fearing they would add a second murder, chooses to take part instead.
Chur stood firm against those who wanted to replace Moshe Rabbeinu during the sin of the Golden Calf, and as a result, the people killed him. While his death is not mentioned anywhere in the Torah, Rashi on the Chumash does refer to it.
Upon witnessing this terrible crime, Aharon HaKohen was left with an awful choice. He could oppose the populace, just as Chur did. However, he likely would suffer the same fate. Aharon HaKohen was not concerned with his own life. On the contrary, he was ready to die Al Kiddush Hashem. However, his murder, in addition to Chur’s, would be a grievous sin. Indeed, it would be one that K’lal Yisrael could not recover from. As bad as the sin of the Golden Calf was, Klal Yisrael could do teshuva from it. However, there is no teshuva from the multiple murders.
Therefore, Aharon HaKohen decided to become involved in the sin of the Golden Calf