It is ridiculous to call for his death when he has not caused someone else's. IF wikileaks at some point leaked in advance something like invasion plans, troops positions, names of undercover operatives, and the people identified died as a result, then IMO it would turn them into an enemy espionage group and basically it would be morally legitimate to start a 'war' with them. But so far they have just leaked secrets from past events, the worst they are guilty of under the law is theft, and you don't execute thieves ever AFTER they are found guilty by a court of law. And that's ignoring the whilstleblower defence for legitimate leaks.
It is a disgrace that politicians are calling for someone to not only be judged guilty of a crime without trial or charge, but to be actually killed without even a chance to defend their actions in front of a jury. Isn't that pretty much the definition of un-American? If anything, the ones who should be executed are the treasonous politicians who are advocating things that the Founding Fathers started the revolutionary war to stop.