I had never heard any specifics about the 1st Marquess of Argyl but it does look like something vaguely resembling the law was respected in his regard.
He was acquitted on the initial charges against him. New information came about damning him, he was then beheaded before Charles II had actually signed the death warrant. That’s obviously legally improper, but it seems that might be more on the people carrying out the execution than the King himself? And it does seem as though some matter of law was followed. Google and Wikipedia are sadly sparse on specifics so I’d have to do more digging.
The thing about most of the early Stuart monarchs and the Tudor monarchs before them is they did have the power to have people killed more or less at will. However, that power was politically checked at various times, and you will note those monarchs more often than not used the guise of the law to carry out executions. Often in show trials of the worst kind, with pure liars testimony and entirely fabricated physical evidence, but the monarchs had enough respect for the law to do a show trial.
If an individual was politically connected enough, that could also often save them from execution in its own right.
Those monarchs also sometimes had their agents do murder in their name. However, it was clandestine, and if ever questioned the monarch’s involvement was strictly denied. Many of the individuals that died during the rebellions in the north of England during Henry VIII’s reign were summarily executed without trial. However, if ever pushed it could always be said that was something done without Henry’s prior approval or knowledge.
Even back to the 1100s Kings seemed to use such methods to kill people. Henry II’s knights that murdered Thomas Becket for example were technically acting illegally and swore it was not on the orders of the King–history is still unsure on that one.
The difference for me I suppose is that a Roman Emperor had various offices that effectively made up the legal framework of their power. One of those offices was *Tribunitia Potestas(*tribunician power.)
Historically Rome was ran by various magistrates. The chief magistrates were the two Consuls who as most know ruled concurrently and could veto one another. Early in the Republic, during a war, the plebeians revolted and demanded the right to elect their own magistrates. (Plebeians could be magistrates, and some were, but the system heavily favored patricians holding magistrate office.) The patricians granted the request and tribunes came into existence. Tribunes could only be plebeians, and were elected exclusively by plebeians. Tribunes had vast authority to veto and stop action, in the general craziness of the time their veto power only had authority during their physical presence. Anything they vetoed could be resumed and executed the moment a tribune had left, but their power to veto and establish criminal proceedings gave them real power.
Interestingly the chief magistrates of Rome, the two Consul’s did not have the power to order capital punishment, and in fact within Rome no citizen could be put to death by a Consul without a trial.
In terms of the State, a tribune had less power than the two consuls, but the power that a tribune possessed had some unique properties. Firstly, it was a capital offense to interfere with the actions of a tribune. A tribune could use any degree of physical force to stop anyone from interfering with his actions. A tribune could also condemn someone to death for interfering with their actions.
When Augustus became emperor he consolidated various offices to essentially create the position of Roman Emperor. Starting with Augustus Roman Emperors held the power of a tribune, the novel thing was they held it without being a plebeian (as Roman Emperors tended towards the Patrician class.) They were also consuls and the official leaders of the Empire’s religion. History tends to judge the beginning of an Emperor’s reign as the point in which he assumed the powers of a tribune (when orderly, a Roman imperial transition tended to involve a successor gradually assuming the various offices that made up the position of Emperor.)
What all of this is to say under the laws of Rome, if the Emperor felt you were interfering with his actions, as the greatest and most powerful tribune, he could legally and lawfully have you killed and there was no legal veto from any party.
True power of life and death.
I do not know how far back in English history you have to go to find a time when that was legal. As King, if an English King of the 1200s or 1300s said “fuck that guy, I want him dead, now” I’m positive someone would oblige him and also positive the King would suffer no repercussions. However, would it be technically legal? For a Roman Emperor their acts genuinely were in that regard.