Then Jacob was greatly afraid and was distressed (Bereishis 32:8)
Rashi comments in the name of Chazal that Yaakov was distressed for two separate reasons: First, that Yaakov would be killed and secondly, that Yaakov would kill others. The first answer that perhaps he would be killed is readily understood: He was afraid for his life and therefore he was distressed. But why would he be distressed if he was forced to kill others? Obviously, if Yaakov was attacked, he would be obligated to kill his aggressor in self-defense, as Chazal teach us “Someone who comes to kill you, you should beat him to it and kill him first!”. Therefore, if Eisav attacked Yaakov, it would be correct for Yaakov to kill Eisav, and there would be no need for Yaakov to be distressed.
One of the answers given to this question is that the whole purpose of Yaakov taking the title of “first-born” was in order to serve as a high-priest in the Beis Hamikdash. The halacha states that any kohen who has killed another man is unfit to give a priestly blessing, so therefore he realized that he might miss out on his purpose. However, this answer is challenged with a halacha says that if a kohen killed someone when the circumstances were beyond his control, he would be allowed to give the blessings.
There is another answer which is given which is truly something to ponder. I would like to preface this answer with a conversation I once had with someone. This person worked in Hatzalah and decided to resign because he felt his children would not have the proper reverence for Shabbos if they saw their father driving on Shabbos, week in and week out, even though they knew that it was a mitzvah for him to do that. This idea is echoed in the Ohr Hachaim (Devarim 13:18), in regards to those who have to put someone to death, due to the city worshipping idols.. The Torah gives those people a special blessing of “v’richamcha” – and you should be given mercy – because killing, even for the most noble of causes, must have a negative effect (see Tosfos Ri Hazakein Kiddushin 82a in regard to being a Shochet).
Here too, Yaakov was concerned that if he had to kill Eisav, it will affect him negatively and affect other areas of his service of Hashem.
People tend to think that if something is permissible it cannot have a bad effect – only things which are forbidden affect a person negatively. Whether discussing the first or second answer, it is clear that the Torah’s perspective negates this concept. There are those righteous individuals who would pray that they should never find themselves in a situation in which they have to do a mitzvah that may put them in jeopardy of losing some kind of a lofty level that they have already acquired. Yet even though that this is true, if Hashem sends to a person a situation that there is such danger, that person would be commanded to do what is necessary.
May we all merit to have to do mitzvohs which generate within us positivity.