The Shulchan Aruch states that a kohen who killed a person even accidentally לא ישא את כפיו, is disqualified from reciting Birchas Kohanim (“the priestly blessing”).
There were soldiers in the IDF who were kohanim who fought in battles against the armies of Egypt, Jordan and Syria when they battled Israel. In the course of the battles, they injured and killed enemy soldiers. R' Ovadia Yosef was asked if these kohanim have the status of a kohen that killed someone where they would be unable to bless the Jewish People with Birchas Kohanim.
R' Ovadia said that it is fitting to encourage such kohanim and praise them for the act and he allowed them to give the priestly blessing without a doubt.
Elsewhere, R' Ovadia was asked by a soldier who was a kohen that was standing at his post protecting against dangerous Arabs when he realized some of them approaching closer to his post. He then initiated a battle that killed several of them. He asked R' Ovadia if he can continue to fulfill the mitzvah of Birchas Kohanim. After saying that he can continue blessing the Jewish People with Birchas Kohanim, he writes that the soldiers who protect Israel are worthy of praise. Their reward is doubled in heaven and they will be blessed with all the blessings in the Torah... The soldiers who put their lives on the line to save Jewish lives deserve honor and respect.
Here is another episode, this one occurring in Europe, where this issue arose. There was a charitable man who was meticulous in fulfilling mitzvos who davened with R' Oshry and attended his daf yomi shiur. On Yom Tov, this man, who was a kohen, performed Birchas Kohanim. During one of the Yomim Tovim, a man who returned from Kovno told R' Oshry that this kohen shouldn’t bless the people because he once killed someone.
After Yom Tov, R' Oshry visited the kohen to hear the story. The man recounted: “I used to peddle merchandise from village to village through back roads and forests and carried a loaded revolver for self-protection. A widow, who was robbed of the last remnants of her property by a Lithuanian non-Jew, requested from me ‘you know my bitter situation and that I am without any support. Please go and threaten him with your gun and tell him to return the poor widow’s property, and that if he refuses, you will shoot him.’ Since I felt I may be able to help her, I went to this person’s house and withdrew my revolver. The man was unimpressed and even got angry that he tried to grab the revolver.” They wrestled, each trying to get his hand on the revolver, and in the struggle, there was a gunshot that killed the Lithuanian. “I still don’t know how the trigger was pulled or who pulled it. I have no recollection of pulling the trigger since the revolver wasn’t in my hand.”
R' Oshry ruled that this death not be considered an accidental murder but manslaughter because he never intended to kill this non-Jew but rather just to intimidate him. Also, even if his finger was on the trigger, he was in danger for his own life. Furthermore, even if the kohen killed him deliberately in the course of the fight, he wouldn't be liable because it had been done in self-defense. Since the kohen regretted his action and accepted penance for having indirectly killed a man, all the views in halacha concur that he is allowed to raise his hands in Birchas Kohanim.
