The parsha begins with Korach leveling a criticism against Moshe and Aharon: "The entire nation is holy and Hashem is among them, and why do you exalt yourselves over the congregation of Hashem?" (Bamidbar 16, 3). Moshe responds, "In the morning Hashem will make known the one who is His own and the holy one." Rashi cites the Medrash which explains the intent behind Moshe's choice of words. He was telling Korach that Hashem set definitive boundaries in the world. Just as he does not have the ability to change morning into night, so too, he does not have the ability to change Hashem's appointment of Aharon as the Kohein Gadol.
Rav Wolbe (Shiurei Chumash) explains that the Medrash is informing us that the appointment of Aharon as the Kohein Gadol was a "creation" no different than the creation of day and night. When Hashem places a person in a specific position, He literally molds the person to fit the position. There is no possibility to change this mold.
This is a concept that applies to each and every one of us. Hashem created each person with a unique character and there is no way to alter this initial character. Chazal tell us that one who is born during the rule of the mazal of Ma'adim, will invariably spill blood. There is no way for him to change this innate characteristic. Nevertheless, continues the Gemara, he has the ability to channel the characteristic to a specific occupation that suits him. He can be a Shochet, a Mohel, a blood letter or a murderer. These are options that span the entire spectrum of occupations. A Mohel spills blood for a vital mitzvah, a murderer spills blood in sin, and the Shochet is somewhere in between. A Shochet also performs a mitzvah, however, it is not an obligatory mitzvah to slaughter animals being that one does not have to eat meat.
Likewise, Chazal tell us that when Shmuel Hanavi came to anoint David as king, he was concerned because David had a red complexion (similar to Eisav) which indicated a natural tendency to spill blood. Shmuel was relieved when he noticed David's fine-looking eyes, since they signified that he would spill blood only with the consent of the Sanhedrin - the "eyes" of the nation.
Every person has certain innate tendencies that cannot be uprooted. The possibilities for channeling these characteristics are numerous. The choice is ours. Why use it in a destructive manner when you have the ability to use it in a constructive fashion?