(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
1. The Shulchan Aruch (O.C. 128:35) rules that if a kohen killed someone, even by accident, he is not allowed to recite the priestly blessings. The Mishna Berura explains the reason for the disqualification is because “ain kateigor na’aseh saneigor”, it would be inappropriate for the hands that have taken a life to give a beracha.
2. The Shulchan Aruch rules that teshuva (repentance) does not help and he remains disqualified. While the Rama writes that after repentance he is permitted to recite the birchas kohanim.
3. According to the Rama a kohen who murdered someone may recite the priestly blessings if he repented. The Biur Halacha cites an interesting argument among the poskim as to the parameters of the ruling of the Rama. Some poskim feel that the lenient ruling of the Rama was stated only for accidental murder. However, if the kohen killed someone on purpose (meizid) he is disqualified even if he repented while other poskim apply his ruling to all scenarios, even purposeful murder. The Biur Halacha writes that it is difficult to know what the correct ruling is, and therefore, if the kohen ascended to recite the blessings we should not stop him.
4. Rav Ovadia Yosef zt”l (Shu”t Yechave Daas 2:16), the leading sefardic posek, writes that if a kohen killed someone against his will “b’ones”, the example given- a kohen who is driving a car within the speed limit and a child jumps in front of the car, (the death is considered b’ones because the driver did everything within his power to prevent it), he is allowed to recite the priestly blessings if he repented.
5. The Magen Avraham (128:53) cites the opinion of Rav Levi Ben Chabib that the hallacha applies even if there are no witnesses to the murder. As long as the kohen knows that he killed someone he may not recite the birchas kohanim.
6. Rav Moshe Feinstein zt”l (Igros Moshe Y.D. 2:158) maintains that if the kohen is a soldier in the army and he killed someone during war time he is unequivocally allowed to recite the priestly blessings. Since the killing is in self defense he is not disqualified. As it says in the Gemara (Brachos 62b): “If someone attacks you to kill you, then you must kill him first.” A similar sentiment was expressed by Rav Ovadia Yosef zt”l -who was discussing the Israeli army as opposed to Rav Moshe who was discussing the Russian army- Harav Yosef writes that these soldiers are fulfilling a mitzvah by protecting the Jewish people and the Jewish land and therefore may bless the Jewish people.
7. If the kohen is unsure whether he killed someone, the Maharlnach writes that he is not allowed to duchan. We treat a doubtful murderer as a certain murderer. However, after analysis, it becomes clear that this question is really based on a machlokes rishonim dealing with a much broader topic than the hallachos of a murderous kohen, as we shall explain.
The source that prohibits a kohen who murdered someone from reciting the priestly blessings is a verse in Isaiah. And we know that when a doubt arises in a biblical hallacha we are machmir (ex. if one is unsure that he ate matza on Pesach he must eat the matza again to be sure). However, when it comes to rabbinical laws, we are lenient in doubtful situations (if one is unsure whether he recited the blessings on food, he may continue to eat without a blessing and need not repeat the blessing). Now the question arises about what is the proper approach in halacha when a doubt arises with a law found in the Navi (Divrei Kabbala). Is it deemed like a biblical law and we should act stringently or like a rabbinical law and we should act leniently? This is actually a machlokes among the rishonim.
The Rashba (Shu”t Hamiyucheses 263 and the Rashash (Shu”t 397) both feel that when a doubt arises regarding a law learned from Navi we rule leniently as we do with rabbinic laws. The Kovno Rav, Rav Yitzchok Elchonon Spector zt”l (Beer Yitzchok Y.D. 1 note 6) wrote that the Ran agrees with the opinion of the Rashba. A similar view can be found in the Aruch Hashulchan (Y.D. 110 note 105).
However, the Netziv (Haemek Sheila Sheilta 35:2) proved that Rav Achai Gaon and the Bahag both disagree and they feel that a law which has sources in Navi has the same law as a biblical law. Similar views can be found in the Turei Even by the Shagas Aryeh (Megila 5b), Shu”t Toras Chesed (O.C. 38 note 8, this also seems to be the view of the Rambam see Mishna Berurah O.C. 692:16).
Now our discussion (a kohen who is unsure whether he killed a person) is dependent on the opinions of the rishonim. For our case, there is a doubt involving a law (namely that a kohen who killed someone is not allowed to duchan) which has a source in the Navi and according to the Rashba (and Rashash etc.) we should be lenient and allow him to duchan, unlike the Maharlnach. However, according to the Rambam (and Bahag etc.) we should be stringent and prevent him from reciting the blessings, in agreement with the Maharlnach. Indeed the Maharsham (shu”t 5:30) cites the ruling of the Maharlnach and notes that according to the Rashba we should allow him to recite the priestly blessings. For practical halacha, a rabbi should be consulted.
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