At the beginning of Parshas Masei the Torah lists the forty-two different places in which Klal Yisrael camped during their travels through the desert. Most of the places are mentioned without comment. One notable exception is Hor Hahor. The Torah abruptly stops its quick-paced account of the travels and dwells on the fact that Aharon Hakohein died on Hor Hahor. Moreover, the Torah adds that Aharon died on Rosh Chodesh Av. Why does the Torah elaborate on the death of Aharon in the middle of the list of travels? What is the significance of the fact that Aharon died on Rosh Chodesh Av?
At the end of the parsha the Torah discusses the laws of a murderer (rotzei'ach). One who kills intentionally is put to death, and one who murders accidentally is required to run to an ir miklat (city of refuge) to atone for his sin and to escape the clutches of the go'el hadam, the relative of the victim who might want to take revenge. The posuk says that the unintentional murderer (rotzei'ach b'shogeg) must stay in the city of refuge "until the death of the kohein gadol" (35:25). Why should the death of the kohein gadol allow the murderer to return home?
Rashi quotes from Chazal that the kohein gadol bears some responsibility for the accidental murder because he should have prayed that this misfortune should not occur during his lifetime. The kohein gadol represents the concept of shalom (peace). After all, the original kohein gadol was Aharon who was a lover and a pursuer of peace (Avos 1:12), which is precisely why he was chosen to be the kohein gadol in the first place. Since a kohein gadol represents all of Klal Yisrael when he performs the avodah in the Beis Hamikdash, he must be someone who unifies all of Klal Yisrael and embodies the middah of shalom.
That is why the kohein gadol, more than anyone else, has a responsibility to pray that no murder should be committed by any Jew, because murder leads to the exact opposite of shalom. Certainly, one who kills intentionally acts in a way that runs counter to the middah of shalom. But even one who kills accidentally disrupts the peace and harmony of Klal Yisrael. He brings people to hatred, to feel a need to take revenge. He creates discord and friction. The kohein gadol, the model of shalom, is charged with the responsibility to daven that there should be only peace and harmony in Klal Yisrael during his lifetime.
But why should the death of the kohein gadol be a reason for the murderer to be able to return home? The answer is that Chazal say the death of the righteous serves as an atonement (Yerushalmi Yoma 1:1). When a tzaddik dies, people think about his life and his personality traits. A person can be inspired by the memory a tzaddik and that can cause him to internalize the values of the tzaddik and to emulate his behavior. In that way, the death of the righteous person can be an atonement because his memory serves to motivate people to do teshuva and to improve their own lives.
Similarly, the death of the kohein gadol should cause the rotzei'ach to think about what the kohein gadol represents - the concept of shalom. That will hopefully cause the rotzei'ach to become more committed than ever to follow the model of the kohein gadol, to pursue peace and harmony and be sensitive to the feelings of other people, which will serve as an atonement for his sin of unintentional murder.
It is not a coincidence that Aharon Hakohein died on Rosh Chodesh Av. During the month of Av we mourn the loss of the Beis Hamikdash which was destroyed because of hatred and insensitivity within Klal Yisrael (Yoma 9b). The month of Av is a time to remember the model of Aharon and to appreciate the importance of pursuing shalom.
Perhaps the Torah elaborates on the death of Aharon in the middle of its discussion of the travels of Klal Yisrael because the journey of Klal Yisrael in the desert was guided by the ananei hakavod, the clouds of glory (see Beha'aloscha 9:15-23), and Chazal comment that the clouds of glory were given to Klal Yisrael in the merit of Aharon Hakohein (see Rashi, Masei 33:40). The clouds of glory were a symbol of the Shechina which rests on Klal Yisrael only when they are in a state of peace. As Chazal say, "When they gather together as one unit, and there is peace among them, then He is their king. (see Rashi, V'zos Habracha 33:5). The Torah in Parshas Masei is hinting to the fact that it was Aharon Hakohein, the ultimate pursuer and lover of peace, who was the catalyst that brought the Shechina, in the form of the clouds of glory, to dwell amongst Klal Yisrael and to give them direction in their travels through the desert.
The loss of the Beis Hamikdash created a distance between Hashem and Klal Yisrael. We no longer feel the same closeness to the Shechina. "The gates of tefilla are locked...a wall of iron separates between the Jewish people and their Father in heaven" (Brachos 32b). But this sense of distance is only a reflection of an ongoing lack of peace and harmony within Klal Yisrael. By emulating the actions of Aharon Hakohein, showing sensitivity and compassion toward each other, we draw the Shechina closer to us, and we can merit the fulfillment of the bracha of u'fros aleinu sukkas shlomecha - may You spread over us Your shelter of peace.