Maybe this is just another argument that can't be solved, because it's all about perspective and opinion.
I'm guessing that Dragonborn intended it that way. There's no fun if there's a right answer.
As I said, I'm not talking from Roggvir's perspective, but from mine. If he understood what would come of Ulfric's escape, than he shares the blame.
The exact same thing could be done from the Imperial perspective to justify the execution.
Judge: "Roggvir, do you swear to tell the truth, the whole truth and nothing but the truth?"
Roggvir: "I swear."
Prosecution: "Did you see what happened?"
Roggvir: "No."
Prosecution: "Did you hear what happened?"
Roggvir: "No."
Prosecution: "Yet you claim it was not a murder, but how do you know?"
Roggvir: "Coz Ulfric told me on his way out."
You can sneak back into the embassy the same way you came in, then go backwards until you get near the party area, then decapitate her. And to save Roggvir, I just quickly ran up the moment I first entered solitude and killed the executioner. Took the battleaxe and started rampaging with it and Elemental Fury.