Hashem spoke to Moshe and Aaron in Mount Hor, by the border of the land of Edom, saying.
The Ohr Hachaim asks, why does the Torah tell us that it was at the border of the land of Edom? The obvious explanation is that the reason Aharon had to pass away is because they came close to the border of the evil empire of Edom.
There is a Medrash that explains the possuk כְהִתְחַּבֶּרְ ךָ עִם אֲחַּזְיָּהוּ פָּרַּץ ה' אֶת מַּעֲשֶיךָ – Because you have joined yourself with Ahaziah, Hashem has made a breach in your works. Just because they attached themselves to Edom, they lost a Tzadik. Hashem told Moshe to tell Klal Yisroel that they were at the edge of the land of Edom, and that was why they lost Aharon Hacohen. They should learn their lesson and keep far away from evil in the future.
Although Aaron Hacohen was destined to pass away because of that which happened at the Mei Merivah, he could have lived somewhat longer if not for their having attached themselves to Edom.
The Ohr Hachaim offers another explanation of this possuk. The possuk (Hoshea 2:17) says וְנָּתַּתִּי לָּהּ אֶ ת כְרָּמֶיהָּ מִשָּם וְאֶת עֵמֶק עָּכוֹר לְפֶתַּח תִּקְוָּה וְעָּנְתָּה שָּמָּה כִימֵי נְעוּרֶיהָּ וּכְיוֹם עֲלוֹתָּהּ מֵאֶרֶץ מִצְ רָּיִם - I will give her vineyards from there, and the valley of Achor for a door of hope; and she will respond there, as in the days of her youth, and as on the day when she came up out of the land of Egypt.
The Medrash says that the future redemption will be unlike the original entry to Eretz Yisroel. The first time we entered Eretz Yisroel, we did not have Moshe and Aaron with us, but at the future redemption, Moshe and Aaron will be there along with us.
The possuk now means that Edom’s border, meaning the edges of Edom’s existence, have been decreed from on High to exist until the future redemption. Hashem did not want Klal Yisroel to fight and win a war against Edom until Moshiach comes. Aharon passed away at the border of Edom, to show that when the end of Edom comes, Aaron will return and enter Eretz Yisroel triumphantly. Aharon had to die now, because Hashem had set the borders of Edom until the end of days, and it was not yet time for Aaron to enter Eretz Yisroel.
