"A jagged shard struck Muradin and seemingly killed him. Arthas discarded his hammer, Light's Vengeance, and rushed to heal his friend. The Lich King, through Frostmourne, began whispering to him, and Arthas left Muradin whom he thought was lost, and took hold of the blade."
"With his soul forfeited to the Lich King, Arthas ran off into the frozen wastes of Northrend."
"When the weapon broke free, Muradin was struck by a rogue shard of ice, rendering him unconscious. Arthas moved to help Muradin, but was dissuaded by the call of Frostmourne in his mind."
Sounds like he was under control of the Lich King.