100:21 If a kohein killed a person deliberately, he may not duchen, even if he repented. (Some authorities disagree. If a kohein who killed deliberately and repented goes up to duchen, he is not told to go down – Bi’ur Halacha 128:35 s.v. afilu asah teshuva.) If he killed by accident and repented, he may duchen. Similarly, an apostate who repented may duchen. If a kohein married a divorcee or a woman who had performed chalitza (or other women prohibited to him – Mishnah Brurah 128:147), or if he rendered himself unclean by a dead body for which he was not permitted to do so, he may not duchen until he has repented by following the instructions of a Torah authority. Other transgressions do not prevent a kohein from duchening. A chalal (the offspring of a kohein and a woman he is not permitted to marry) may not duchen.
100:22 An onen (one who is waiting to bury a relative) does not duchen. A mourner within the twelve months for a parent or within the thirty days for other close relatives does not duchen. He needs to leave the shul before “R’tzei” until after duchening is finished. If there aren’t two other kohanim in shul, then the kohein in mourning may duchen within the twelve months for a parent or the thirty days for other close relatives. A mourner during the week of shiva, such as one who buried his relative on chol hamoed, may not duchen even if there aren’t two other kohanim in shul. (If he didn’t go out and they called him, he must go up because not to do so would violate a positive commandment – MB 128:157.)