Brachos 32 - A Kohen Who Killed May Not Duchen

When a Kohen says birkas kohanim he must be filled with love in his heart for every Jew. Not only for klal yisrael on a communal level but also for "Reb Yisrael" on an individual level. The poskim say that if the kohen doesn't actively feel love for every Jew in shul it is better that he not duchen.

His feelings must extend not only to the neshamos of fellow Jews but to their physical bodies as well. This is expressed by the "nesias kappaim" - the spreading out of the hands, which are the vessels of physical activity in this world. They are in essence saying "We love you and are concerned with your welfare". If a Kohen killed a fellow Jew, even by accident, it is indicative of a lack of love towards other Jews. He is thus not worthy of saying birkas kohanim.

עין איה עמוד 149