Tbh I've never had it against Jamie that he killed the Mad King. He had this nickname for a reason. It's the things he has done later that condemn him in my eyes. The bath scene was great acting though.
They've really ruined book Robb in the show, IMO. First they made him seemed like a spoiled brat by marrying Talisa, thus screwing over a marriage pact, just by "being in love". Whereas he was devastated in the books having learned of his brothers "deaths" and injured in battle, thus succumbing to a certain highborn woman and marrying her not just for love, but because he took her maidenhood away without marriage and wanted to save her honor, by you know, marrying her.
Now they handled this Karstark execution without any sort of honor. In the books Karstark practically forced him to behead him if he was a true king and insisted he swing the sword. This was closer to the books than the Talisa business, but still made me cringe how easily he made the decision in the show..There was a lot more complexity to Robb's actions in the books as compared to the show. :-/
Book version of Robb seems more fleshed out, as it should be, but I still dig show Robb too, get what he is doing even if they don't always clarify it. He has done an immense job IMO. Better then his dumbass father.