Halakhah sur Isaïe 1:15

A kohen who has killed may not perform Birkat Kohanim, as it is written: “When you spread your hands, I will turn My eyes away from you… your hands are full of blood” (Yeshayahu 1:15; Berakhot 32b). The kohen’s job is to increase kindness and life, like Aharon, who loved and pursued peace. A kohen who has killed has damaged the core of his priesthood. Thus, a kohen who unintentionally ran someone over with his car may not perform Birkat Kohanim. The poskim disagree about whether he may repent by fasting, tzedaka, and resolving not to sin again would enable him to once again perform Birkat Kohanim. In practice, a kohen who undergoes such a terrible experience must go to his rabbi to receive personal instruction about what to do (Peninei Halakha: Prayer 20:11).