I am not sure why this topic intrigued me, but I really hope it is not halacha l'ma'aseh for too many people (aside from fighting in the Israeli army).
The gemara in Berachos 35a says that a kohein who kills can't duchen. Tosafos in Sanhedrin 38b brings 2 reasons why a kohein who kills can do the avodah but can not duchen. Either it is a chumrah or because of the rule "ein kateigor na'aseh saneigor", since the kohein used his hands to kill someone, he can not use his hands to bless Klal Yisroel.
There are 2 questions raised regarding this second teretz of Tosafos.
1) The Aruch LaNer asks that the kohein gives a beracha with his mouth, not with his hands. So how come we say "ein kateigor na'aseh saneigor"? He answers that we see from here that raising the hands is a cheilek of the mitzvah. In fact I saw brought down a rayah to this from the Shulchan Aruch HaRav. He paskens that the beracha is said before the hands are raised because the beracha must be before the nmitzvah. We see from here raisng the hands is part of teh mitzvah.
2) The gemara in Rosh Hashana says that "ein kateigor na'aseh saneigor" only applies inside the Besi HaMikdash. That is why the kohein gadol does not where bigdei zahav on Yom Kippur in the kodesh kedoshim, because it reminds HKBH of the eigel hazahav and "ein kateigor na'aseh saneigor". However, a cows horn would not be pasul for shofar because of this reason since the shofar is blown outside the Beis HaMikdash. The achronim ask, if this is true why does "ein kateigor na'aseh saneigor" apply here-a kohein duchins outside the Beis HaMikdash. The answer given (I believe it is a Pri Chadash) is that when the mitzvah is being done with the same object used to do the aveirah, then "ein kateigor na'aseh saneigor"applies even outside the Beis HaMikdash. So here the hands that killed are teh hands that are giving the beracha.
Nafka Mina in Tosafos
1) There is a machlokes between the Mechabeir and Rema whether teshuva allows a kohein to duchen. The Mechabeir is machmir and the Rema is meikil. (The Biur Halacha seems to say we should be machmir for the Mechabeir unless it is a real oneis). I saw that this could be toloi on the 2 answers of Tosafos. If you hold it is only a chumrah not to duchen, then once a kohein does teshuva he can duchen again. However, if the reason is because "ein kateigor na'aseh saneigor", then teshuva might not help. We know the B'nei Yisroel did teshuva for the eigel, yet a kohein still doesn't wear gold in the kodosh kedoshim. (you could be docheh this and say maybe the teshuva wasn't completely accepted)
2) another nafka mina is that the rule of "ein kateigor na'aseh saneigor" could be a sevarah d'o'reisa. While chumrah is only m'd'rabanan.
The Piskei Teshuvos (Siman 128) discusses different issues (car accident, army etc). A couple of interesting questions are.
1) A kohein is drafted by his country and kills in battle.
Rav Moshe paskens that the kohein can duchen. The Mishna Berura paskens that even if someone violates yahrog v'al ya'avor and kills someone, since he was an oneis he can duchen. Therefore, Rav Moshe says kal v'chomer if you are conscripted into the army, you are an oneis and you can still duchen. I understand from Rav Moshe's comparison to yahrog v'al ya'avor that he is referring to a case of someone who kills a fellow Jew in battle.
2) Israeli soldier kills an enemy soldier
Rav Ovadiah Yosef says he can certainly duchen. First of all it is not clear if this issur applies to killing a nochri. Second of all you are defending yourself and klal yisroel and you are at worst an oneis. He does not discuss killing a fellow Jew through friendly fire.
The Piskei Teshuvos brings down that the Sefardishe poskim go with the Mechabeir and teshuva deos not work while the Ashkenazi poskim follow the Rema that teshuva does work. He does bring down that if you killed b'oneis like in a car accident where it totally wasn't your fault, then even Rav Ovadiah Yosef will allow you to duchen with teshuva.