Legends of the Broadsword: "Traitor" Colquhoun
The whole concept of the honorable challenge is predicated on the assumption that one's opponent is also a man of honor. A man who had lost his honor had nothing else to lose but his life, and challenging such an opponent could prove to be foolhardy.
During a feud between the MacFarlanes and the Colquhouns, a servant of Sir Humphrey Colquhoun was captured by the MacFarlanes, and compelled at sword-point to betray his master. He was utterly despised for the rest of his life, and compelled to go everywhere armed for fear of his own clansmen, who called him "Traitor" Colquhoun and shunned both him and his family.
When a wandering swordsman came into the district looking for a challenge, someone thought it would be a good idea to send him after the hated Traitor. The man came up to the door, and Traitor came out.
"What do you want?" he asked.
"A man to fight me with the sword or pay a ransom," the swordsman said, having been told that Traitor was the local champion. Traitor went back into his house without a word, returned with a drawn sword, and ran the man through before he could react. The prizefighter never even had a chance to draw his weapon.
(History of the Clan MacFarlane by Cynthia Maria Jones and Atwood Little, 1893)