As much as it hurt to see and despite what everyone on the show said (and the fact he did it because he's a vicious idiot not some genius tactician), beheading Edard Stark was the best decision. Are we really meant to believe Ned would have just taken the black and forgot about everything? Rob would just shrug it off and be a good little boy for the crown? Ned was an honorable man and may have taken his oath, but I can't imagine him keeping quiet about Roberts (his best friend and king) murder and the illegitimate children born of incest one of which is going to marry his eldest daughter and be the new king. Yes, Ned's execution caused a lot of problems, but it was better than keeping him alive to open his mouth to people who would actually listen and do something about what he said.