Holy Hands and Holy Stones
One of the differences between the Beis Hamikdash and the Mishkan, was that the Mizbeach where the Karbanos were offered was made of stone not gold. In Parshas Yisro, the Torah commands us that when building a stone mizbeach, the stones cannot be cut with metal. Chazal explain that metal is used to make weapons which kill, and thus is not fit to be used in preparation of the mizbeach which brings about atonement, and consequently saves people.
In a similar vein, the Halacha (O.C. 128;35) states that a Cohen who murdered cannot Duchen (give the priestly blessing). The Mechaber rules according to Tosfos that this applies even to someone who has repented, because the issue at hand is not whether the Cohen is a righteous person, but whether the hands that murdered can be a conduit for such a holy and spiritual act. The Rama rules leniently if a Cohen has done the proper teshuva, so as not to dissuade people from repenting.
Regarding a doctor who inadvertently caused a patient to die, the NIshmas Avraham explains that he can certainly Duchen according to the Rama (assuming he did the proper teshuva). Even according to the Mechaber, this is not an ordinary instance of “murder” for the doctor acted within the parameters of Halacha and his intention was to do a mitzvah. The fact that he acted with noble intentions removes the fault associated with an accidental murder and has the Halachic status of “Ones” for which a Cohen can still Duchen.
Furthermore, if a Cohen performed a non-Halachic abortion, he may still Duchen. This is because while what he did was asur, aborting a fetus is not punished with death in Halacha, and is therefore more lenient.
Aside from the Halchic implications, these Halachos remind us that if we want to be a conduit of Kedusha and mitzvos, we must ensure that we don’t implicate ourselves and our body with thoughts and behaviors that are contradictory.
Adapted from medicalhalacha.org