Looking for this bl(?)

I don’t remember if it was shounen ai or not (def not yaoi). One dude has black hair and the other one has blond hair. Older one killed younger one’s dad (whole family?) when he was young (his first mission). Later in life, younger one’s is seeking revenge and somehow older one ends up becoming the younger one’s mentor/caretaker figure. Older one knows he’s the one who killed the dad, younger one looks up to him. Older one feels guilty and thinks about telling him the truth but he can’t muster up the courage to say it. Don’t remember much more. Probably action like plot, don’t think it was a fantasy like world with magic and stuff. Not a modern world with today’s technology, like ”noble society” vibe.