Answer: The Gemara (Berachot 32b) states: “Rabbi Yochanan said: A Kohen that murders another may not recite Birkat Kohanim, as the verse states, ‘And when you spread out your palms I shall hide my eyes from you; even when you make many prayers, I will not hear, your hands are full of blood.” This verse implies that Hashem does not wish to hear Birkat Kohanim from someone who has killed another person. The Poskim, as well as Maran Ha’Shulchan Aruch (Chapter 128, Section 35) rules likewise.
The Sefer Tzedah La’Derech writes that even if the Kohen killed a non-Jew, he should also not recite Birkat Kohanim. Other great Poskim concur. On the other hand, other Poskim disagree and write that although there is a Torah prohibition to kill any human being, nevertheless, a Kohen only becomes invalidated from reciting Birkat Kohanim if he kills a fellow Jew.
Maran Rabbeinu Ovadia Yosef zt”l writes in his Taba’at Ha’Melech (Ma’or Yisrael on Rambam, Chapter 15 of Hilchot Tefillah) that he was asked by a combat soldier who was a Kohen and was stationed at an IDF post to guard against terrorists. As he stood guard, he noticed some Palestinians coming close to the post and he immediately opened fire, killing some of them while the remainder fled. This soldier’s question was, from that day on, would he be permitted to fulfill the Mitzvah of blessing the congregation with Birkat Kohanim?
Maran zt”l ruled that he was permitted to recite Birkat Kohanim since according to all opinions, this scenario certainly qualifies as a time of war and in such circumstances, the Gemara (Berachot 62b) states, “If one comes to kill you, rise up and kill him first.” On the contrary, if this soldier would have hesitated in opening fire, who knows how many Jews would have been killed by these blood-thirsty terrorists. Indeed, if he would have done so, the soldier would have transgressed the negative commandment of “Do not stand idly by your fellow’s blood.” Hashem granted him the merit to eliminate the terrorists before they had a chance to inflict harm and he shall be greatly rewarded in Heaven for his courageous act.
Maran zt”l adds that clearly, Kohanim who serve in the IDF and eliminate their enemies in protection of their brethren are worthy of great praise and thanksgiving and they shall be blessed with all of the blessings in the Torah. When we find that the Poskim write that a Kohen who killed another in battle should not recite Birkat Kohanim, this only applied in specific situations where Jewish soldiers were forcefully drafted to fight in the armies of their respective countries and they would fight against another nation with whom they shared no previous aggression. However, regarding a battle meant to protect the Jewish nation against cursed terrorists that cruelly wish to murder all of their men, women, and children and the soldiers of the IDF put their lives on the line in order to save Jewish lives, soldiers who participate in such battles deserve honor and glory. The Kohanim among these soldiers may certainly recite Birkat Kohanim and they shall be blessed from Above, as the verse states, “And I shall bless those who bless you.” May Hashem protect them and us always, Amen.