He is the King's Hand, damn it. He'll arrest Jaime!

"Jaime Lannister," declared Eddard grandly, "you are ordered to stand down in the name of the King..."

"Mmmm... No, no, I don't think I will," answered the Kingslayer, charging forward. The Lannister horsemen rode through the shocked Stark guards, slashing and slaying. Eddard drew his sword and desperately tried to rally his men, hoping they could turn the tide...