Bronn from Game of Thrones put a crossbow and aimed it at both Jaime and Tyrion, Hand of the King, demanding a castle.
Does this make sense?
Usually, robbing is done to steal wallets or things whose property is far away from governments' control.
Also how to enforce the deal? I don't kill you, I got Riverrun. After he's not killed, Tyrion can always report the crime of coercing the Hand of the Queen to the queen and get Bronn killed instead.
Imagine I put a gun on the US president and demand the White House. It wouldn't work, right? Even if the president says yes, then what?