The Gemara teaches in Masechet Berachot (32) that a Kohen who kills somebody, even accidentally, is then disqualified from reciting Birkat Kohanim (the priestly blessing in the synagogue). This Halacha is brought by the Shulhan Aruch (Orah Haim 131), who emphasizes that this applies for the rest of the Kohen’s life, even if he repented. Although Teshuba (repentance) is effective in erasing a person’s guilt, nevertheless, the Kohen remains forever disqualified from conferring Birkat Kohanim upon the congregation.
Already the Zohar, in Parashat Pinhas, raises the question of how to reconcile this Halacha with the story of Pinhas, who was rewarded for his act of zealotry by receiving the status of Kohen for himself and his descendants. Hashem had delivered a deadly plague upon Beneh Yisrael when they sinned with the women of Moab, and Pinhas ended the plague by killing two public violators – Zimri and Kozbi. He was rewarded for his act by receiving "Berit Kehunat Olam" – the status of Kohen for all eternity (Bamidbar 25:13). How, the Zohar and others ask, could Pinhas specifically become a Kohen after intentionally killing two people, if a Kohen loses his status if he accidentally kills a single person?
Intuitively, we might have answered by distinguishing between a Kohen who kills, and a non-Kohen who kills. A Kohen who kills has defiled his priesthood and thus loses this status, but Pinhas was not a Kohen at the time he killed, and so he did not defile his priesthood. It seems, however, that the Zohar and others who raised this question felt that to the contrary, if somebody who is already a Kohen loses this status by killing, then certainly one cannot attain the status of Kohen by killing.
The Seror Ha’mor (Rav Abraham Saba, 1440-1508) answers this question by explaining that Pinhas’ case was exceptional, given the unique circumstances under which he killed. Normally, the Seror Ha’mor writes, a Kohen who kills becomes disqualified from the Kehuna (priesthood) because even if he kills unintentionally, G-d is angry at Him. Every person is created in the image of G-d, and so killing has the effect of diminishing G-d’s image in the world. Pinhas, however, killed in order to end the plague that had killed thousands. G-d says explicitly that His anger abated because of Pinhas’ act ("Heshiv Et Hamati" – Bamidbar 25:11), and that He rescinded His decree to kill all of Beneh Yisrael. Thus, Pinhas’ act did not arouse anger – to the contrary, it caused G-d to stop being angry – and it saved many thousands of lives, thereby preserving the divine image. As such, he was worthy of the great privilege of the Kehuna.
This also answers a different question – how the Kohanim were given their position in the first place. We know from Parashat Vezot Ha’beracha (Debarim 33:8-9) that the tribe of Levi was chosen as the tribe of Kohanim because after the sin of the golden calf, they heeded Moshe’s instruction to kill the violators. Here, too, even before Pinhas, there were people who killed yet were appointed to the position of Kohanim. The explanation is that, as in the case of Pinhas, the Leviyim assuaged G-d’s anger and saved many lives by killing those who had sinned, and so they not only did not forfeit the privilege of Kehuna, but were specifically rewarded by being named Kohanim.
The Shulhan Aruch writes that an exception to this Halacha is a case where a Kohen killed as a result of his involvement in a Misva. One case where this arises is that of a Mohel, who performs a Berit Mila and the infant then dies, Heaven forbid. If the Mohel is a Kohen, he may continue reciting Birkat Kohanim, and he does not lose his status as Kohen, despite having caused the infant’s death. The Mordechi (Rav Mordechai Ben Hillel, Germany, d. 1298) writes that in such a case, since the death occurred as a result of a Misva, and in any event, it cannot be definitively determined that the infant died because of the circumcision, the Mohel does not lose his priestly status.
This exception, in the case of a Kohen who accidentally killed while performing a Misva, might be relevant to an unfortunate incident that occurred during the COVID-19 pandemic. There was an elderly man who lived alone, and the only person with whom he was in contact was his son, who visited him each and every day and tended to his needs. During the coronavirus outbreak, the father insisted that his son come to visit him every day despite the risk of infection, because he was all alone. The son obeyed his father’s wishes, and came to visit him each day, but it turned out that the son was infected with the virus, and he transmitted it to his father, who, sadly, passed away. The question arose as to whether the son – who is a Kohen – is still permitted to recite Birkat Kohanim, given that he is certain that he transmitted the illness to his father, who had no contact with anybody else in the world.
This question hinges on a number of different factors, primarily, whether infecting somebody with a fatal illness is Halachically considered "killing." From the sources, it appears that the Halachic definition of "Resiha" ("murder") is not limited to directly taking somebody’s life. For example, the Yehuda Yaaleh (Rav Yehuda Assad, Hungary, 1794-1866) writes that if a person took somebody’s life by proclaiming a Name of G-d, this qualifies as "Resiha." He draws proof from the tradition that when Moshe killed the Egyptian taskmaster who was beating a slave, he did so by proclaiming a Name of G-d. The Torah speaks of Moshe killing the Egyptian with the word "Va’yach" (Shemot 2:12), a derivative of the verb "Makeh," which the Torah uses in speaking about the punishment for murder ("Makeh Ish Va’met Mut Yumat" – Shemot 21:12). This would seem to indicate that Halachic "murder" is not narrowly defined as directly striking a person, but includes even indirectly taking a person’s life.
Another possible basis for this conclusion is the law of "Esh" – the liability assigned by the Torah to one who kindles a fire in his property which is then carried by the wind and damages another person’s property. The Gemara discusses how this law introduces the concept of liability even in a case of "Koah Aher Me’urab Bo" (literally, "another force is involved"), where one produces something which another force then carries and ends up causing damage. One’s liability in the case of "Esh" is described by the Gemara as comparable to the liability for damages one causes by shooting arrows ("Esho Mishum Hisav"). Even though the fire is carried by another force – the wind – as opposed to arrows, which travel directly through one’s own force, one is liable for damage caused by his fire just as he is liable for damage caused by his arrows. Conceivably, this is relevant also in the case of a person who transmits a contagious illness. He coughs or sneezes, emitting pathogens into the air, and the wind then carries these pathogens to other people, causing them harm. Quite possibly, he would then be liable for the damages caused, and if somebody contracts the illness and dies, he might be guilty of unintentional murder.
Of course, this is far from clear, and this subject requires further analysis. But the question itself should certainly alert us to the need to exercise extreme caution when it comes to contagious illnesses, and be exceedingly careful to avoid causing others to become ill, even indirectly.
Returning to the situation of the son who infected his father while visiting him, it is likely that since the son was fulfilling the Misva of Kibud Ab (respecting one’s father), this is akin to the situation of an infant who dies as a result of Berit Mila. As we saw, the Kohen may continue reciting Birkat Kohanim, since the death occurred as a result of a Misva. By the same token, perhaps, the man in this case would be allowed to continue reciting Birkat Kohanim, since the tragedy resulted from his performing a Misva.
Summary: A Kohen who killed somebody, even accidentally, loses his status of Kohen, and may no longer recite Birkat Kohanim, even after repenting. It is possible that this would apply to the case of a Kohen who negligently infects another person with a contagious illness from which that person dies. However, if this occurred as a result of a Misva, such as if the Kohen was visiting his elderly father who lived alone and had nobody else to visit him, and infected him, he might nevertheless be allowed to continue reciting Birkat Kohanim. This subject requires further analysis and consultation with leading Halachic authorities.