The Gemara in Masechet Berachot (32) cites Rabbi Yohanan as ruling that a Kohen who killed another human being may not recite Birkat Kohanim (the "priestly blessing"). The source for this Halacha is a verse in the Book of Yeshayahu (1:15): "And when you spread your hands, I will turn My eyes away from you… Your hands are filled with blood." This verse implies that if a person’s hands are "filled with blood," meaning, if he killed another human being, God will ignore the blessing he proclaims by outstretching his hands. The Sages thus inferred from this verse that a Kohen who killed another person should not perform Birkat Kohanim, because his blessing is not acceptable to God.
The Shulhan Aruch (Orah Haim 128) codifies this Halacha, and rules, based on the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), that it applies even in the case of accidental murder. Even if a Kohen killed another person accidentally, and even if he sincerely repented, he is disqualified for Birkat Kohanim. The exception to this rule is where somebody killed another person due to circumstances entirely beyond his control. For example, if a person was driving safely, within the speed limit, and somebody jumped in front of the car, such that the driver had no possibility of avoiding him, the driver may recite Birkat Kohanim even though he caused another person’s death. Since the death occurred due to circumstances entirely out of his control, and he bears no responsibility at all, he is not disqualified for Birkat Kohanim. This is the ruling of Hacham Ovadia Yosef. However, if a Kohen was driving recklessly and as a result killed somebody, Heaven forbid, he may not, according to Sephardic custom, recite Birkat Kohanim, even if he performed sincere Teshuba (repentance). This is in contrast to Ashkenazic custom, which allows one to recite Birkat Kohanim after repenting in a case of accidental murder.
A Kohen who killed an enemy during battle, such as a Kohen who fought in the Israel Defense Forces and killed enemy soldiers or terrorists in war, may continue reciting Birkat Kohanim. This is the ruling of Hacham Ovadia Yosef, in his work Yehave Da’at.
The Shulhan Aruch addresses the case of a Kohen who works as a Mohel and an infant died as a result of a circumcision he performed. In such a case, the Shulhan Aruch rules, the Mohel does not become disqualified for Birkat Kohanim, since the infant’s death could be attributed to other factors. The child was perhaps underdeveloped, ill, or otherwise incapable of surviving the procedure. The Mohel thus cannot be definitively blamed for the tragedy, and he may therefore continue reciting Birkat Kohanim. The Halachic authorities apply this principle to the case of a patient who dies under the care of a physician who is a Kohen. Assuming the physician was not negligent and performed his duties properly, he may continue reciting Birkat Kohanim, as he does not necessarily bear responsibility for the patient’s death.
Summary: A Kohen who killed another person may no longer recite Birkat Kohanim; according to Sephardic custom, this applies even if a Kohen killed somebody accidentally, such as in the case of death caused by reckless driving. This Halacha does not apply, however, if a Kohen killed due to circumstances entirely beyond his control, if a Kohen killed an enemy during battle, or if a Kohen works as a doctor and a patient died under his care (assuming the physician executed his responsibilities adequately).
