The Talmud teaches that a Kohen who has killed someone is disqualified from the Birkat Kohanim. The Shulchan Arukh (O.C. 128:38) rules this applies even to an accidental killing (see the comment of Chatam Sofer, Torat Moshe, Tetzaveh, p. 68a, that Moshe could not serve as Kohen Gadol due to his killing of the Egyptian), and even if repentance has taken place. The Rama, however, adds an opinion that repentance should be accepted in this context. (See Resp. D’var Yehoshua, IV, 54:2, and Resp. Ginzei Yosef, 97).

R. Moshe Feinstein (Resp. Iggerot Moshe, Y.D. II, 158) takes it as obvious that a Kohen drafted into the army at wartime, who kills in that context, remains qualified for Birkat Kohanim. (See also Resp. Hit’or’rut Teshuvah, I, 58, and Resp. Chukei Chaim, IV, 33). Other issues discussed by poskim include a Kohen who commited vehicular manslaughter (See Resp. Teshuvot V’Hanhagot, I, 131; Resp. Minchat David, VI, 106:4; Keter Ephraim, 33); a Kohen who has performed an abortion (see Birkhat Aharon, ma’amar 287); and a Kohen who kills accidentally in the course of performance of a mitzvah (e.g., milah; see Shulchan Arukh, O.C. 128:36; Resp. D’var Yehoshua, IV, 56:6; Marpei L’Nefesh, I, 34:5).

Another issue taken up by some poskim concerns the comparison by the Talmud of one who embarrasses others to one who commits murder (Berakhot 43b, Sotah 10b, Ketubot 67b, Bava Metzia 59a).. Would this comparison be taken literally, to exclude a kohen who embarrasses others from Birkat Kohanim? R. Natan Gestetner (Responsa L’Horot Natan VIII, 2) analyzes the issue, noting the debate among authorities concerning the question of martyrdom to avoid shaming others. He concludes leniently, citing a supporting implication in the Responsa Ralbach (#117, referred to in the Magen Avraham, Orach Chaim 128:153) finding the comparison to be less than total.

He concurrently advances a technical argument, proving that even were the severity of humiliation equivalent to (or worse than, as he notes in the name of the P’nei Yehoshua) homicide, this kohen would nonetheless not be excluded; the category is limited to physical murder, as indicated by Talmud, citing the verse, “your hands are filled with blood.” (Yeshayahu 1:15; see Tosafot, Yevamot 7a, s.v. Sheneamar, who explain that hands used for murder cannot then be effective in prayer. Although a similar conclusion might he reached as to the appropriateness of a mouth that has embarrassed another being used to pronounce the prayer, this is nonetheless not explicitly within the text.) The identical conclusion is reached in a responsum by R. Shmuel Wosner (Responsa Shevet HaLevi, VIII, 172). Nonetheless, R. Baruch Weiss, in an independent analysis of the issue in his book-length treatment of the laws of birkat kohanim, (Birkhot Horai 23:12:9). reaches a stringent conclusion and forbids such a kohen from partaking in the blessing.

R. Gestetner’s conclusion vis-a-vis non-physical killing may be relevant to a question discussed by some concerning one who kills another through casting a mystical spell (see the journal Ohr Torah [Kol Aryeh], II, 91:5, citing Resp. Halakhot K’tanot, II, 98; Resp. Yehudah Ya’aleh O.C. 199; Resp. Hit’or’rut Teshuvah IV, 9).