Chivalry: Not Really About Opening Doors (and Still Quite a Useful Coping Strategy)

So, Lancelot is fighting this knight who insulted him. A few blows in and the rude fellow is on his knees: “Mercy, Sir Lancelot!”
Sir Lancelot stays his hand. He always grants mercy when asked.
However, before he can help the other chap to his feet, a lady rides up. “Lancelot! I beg you a favor.”
“OK…,” says Lancelot. He also always grants favors to damsels.
“Give me that knight’s head! He slew my sister.”
Lancelot frowns. Now he’s caught between two imperatives.
He can’t give the lady her favor and spare the other man.
What’s he going to do?