Even if he didn't tell why, you don't need to be a genius to understand the reason or at least understand that it was a good thing that the King died, he saved Robert a lot of trouble.
Furthermore, after Tywin betrayed the King, where was Ned honor ? Nowhere because Tywin was too powerful to be given a shameful title like Jamie.
Robb has even a worst sense of honor than his father, he made a deal with Lord Frey and broke his word for a woman, killed of his banner men instead of using his brain and jail him.