Ned being killed off early is a key part of the "no one is safe in the game of thrones!" motif. Having a known actor and playing against type (nice guy when usually villain) just underscores the surprise.

There is a similar story about Gene Wilder in Charlie and the Chocolate Factory. He first appears with a limp and then somersaults to show he's fine. The whole point of that scene is to have the audience think "I can never trust anything this guy says again".