Geoffroi de Charny, a 14th century French knight, carried out a revenge killing on a man who betrayed him. He and some companions attacked the guy's estate, dragged him to town, and decapitated him in public. Then they hung his body at the gate.

In de Charny's book on chivalry, revenge killings and feuds are discussed. Whole thing is an interesting read. This sort of thing was common in the Middle Ages, something often forgotten today. de Charny's revenge killing caused him no problems, and was even praiseworthy.

In his book, de Charny say that someone taken prisoner in
a battle should be kept for ransom, and treated with the honor due to their station. But, he includes a caveat. If that person has done you some great harm, or dishonored you, it is acceptable to kill them instead.

De Charny ate dinner with the King of England on the same day he was taken prisoner after a failed attempt at an infiltration and raid. The King was angry with him, because of what had happened, but still gave him accommodations. He ended up a prisoner in England for a while.