Doeg HaEdomi was a Rasha, who told Shaul HaMelech that Achimelech, the Cohen Gadol had rebelled against him, and was Chayav Missa. So Shaul told his two top generals, Avner ben Ner and Amasa ben Yesser to kill them. The Passuk says that they both refused to kill Cohanei Hashem (and Avner was Shaul’s most trusted person who supported Shaul and his Malchus at all costs). So Shaul told Doeg to kill them, and the Passuk says that he killed Achimelech and 85 Cohanim (but they aren’t called “Cohanei Hashem”).
The holy Alshich points out how Avner and Amasa were Tzaddikim and had Rachmanus on Cohanei Hashem, and in all probability they were simply in awe of such giants. But Doeg who ended up losing his Olam HaBah didn’t view them with the same respect, and by him they were called plain Cohanim.
We see the greatness of a Cohen during the time of the Bais Hamikdash. The Cohanim would kill themselves (literally) to be able to do the most-simple Avoda in the Bais Hamikdash, like Terumas HaDeshen (lifting the ashes). One time, two Cohanim were racing up the Kevesh (ramp) of the Mizbeach, and one Cohen was getting ahead, so the other Cohen took a dagger and stabbed him.
