Who killed the priests of Nob when Saul's own guards refused?

Answer: Doeg the Edomite

Doeg struck down eighty-five men who wore the linen ephod and put Nob to the sword. Only Abiathar escaped to David.

“So the king ordered Doeg, “You turn and strike down the priests!” And Doeg the Edomite turned and struck down the priests himself. On that day he killed eighty-five men who wore the linen ephod.”— 1 Samuel 22:18 (BSB)