After Karna's death, Duryodhana appointed King Shalya as supreme commander.
"Only you can kill Shalya," Krishna told Yudhishthira. "Inside him is a demon that feeds on aggression, which means the only way to defeat him is to confront him with love and respect. You alone are capable of this."
So Yudhishthira confronted Shalya on the battlefield with love in his heart, not hatred or anger, thus rendering Shalya's demon powerless. Doing his duty, Yudhishthira then killed Shalya with a single spear-thrust. "Forgive me," he said, weeping, for Shalya was the brother of Madri, uncle to his brothers Nakula and Sahadeva.
Inspired by: Jaya: An Illustrated Retelling of the Mahabharata by Devdutt Pattanaik.
Notes: This story is on p. 278. Pattanaik notes that the story of Shalya's inner demon comes from the Indonesian retelling of the Mahabharata. He also notes that because both Shalya and his son died in the war, the kingdom of Madra then passed to his nephews Nakula and Sahadeva.