Who would slap on the cuffs? For example, assume that he’s currently King and said, as is popularly remembered, “Will no one rid me of this turbulent priest?” and the deed was done. Under current law, would or could he be charged with some crime?
The first problem would be the conflicting accounts of what was actually said:
in reality, there’s no way of knowing precisely what Henry said. Edward Grim, the most influential of Becket’s hagiographers, reports a different exclamation in his account of c1171–72. Grim, who was an eyewitness to the murder, wrote that Henry said: “What miserable drones and traitors have I nurtured and promoted in my household who let their lord be treated with such shameful contempt by a low-born cleric!
The second problem is that The current king has no executive authority, so a putative assassin would have a big problem trying to convince a jury that they were acting under orders.
“Getting rid” of someone does not mean killing them. I suppose that in some circles (organised crime, cartel) it may, but if a frustrated leader said anything like that today, they would probably be given a mug of cocoa and put to bed.
Besides the question of if it was a crime at all, the monarch is immune from prosecution. If Charles III decided today to personally lop the head off Justin Welby, he could still not be prosecuted. The methods for dealing with that situation would be extra-constitutional but easily understood by reference to a previous Charles.
And in a non-royal context we now have precedent for the defence that “I genuinely believed that my interpretation of what I said was what it actually meant whatever anyone else thought and nobody told me that my underlings were doing anything different anyway so I am innocent of all charges which are a politically motivated and illegitimate fraud. So yarboo sucks with knobs on and no returns”.
In general, incitement to violence is a crime. As @ctnguy says there are procedural difficulties with prosecuting the King but if Boris Johnson were to say something like “Dominic Cummings is a real pain in my arse, in the good old days he’d get the Thomas Beckett treatment” and some of his acolytes arranged just such an end, then there would be a pretty clear case for the prosecution. (Although Boris would of course run exactly the defense @PatrickLondon outlines)
… and also exempt from civil as well as criminal proceedings, and from income tax and capital gains taxes, although the Queen voluntarily began to pay taxes in 1992, and always did have to pay VAT on purchases. The monarch (and the monarch alone) is also exempt from having to hold a driver’s license (Prince Philip did have to have one, and voluntarily gave it up after his accident). Queen Elizabeth did have a military driver’s license while serving in the military in 1945, listing her name as “HRH Princess Elizabeth” and bearing her signature, “Elizabeth”. It was the only driver’s license she ever held.
Yes, as I understand going back centuries, crimes were offenses against the crown - so Henry isn’t going to offend himself. That doesn’t seem to make legal sense. besides, he could pardon himself. (The logic as I understand it is that killing someone who is under the control of the crown is an offense to the crown, just as if you’d killed the King’s cow or dog.)
I assume though, killing a cleric is a violation of church law - not sure if it applies to monarchs, or if the church in that case has jurisdiction over non-church actors who commit crimes against them. The murder was also committed on church land. Of course, back then did they have conspiracy laws? I suppose the new Archbishop could send a posse of monks to arrest the king. That would go over well…
There’s the suggestion that Edward II died in interesting circumstances. There was the suggestion a message in the days before punctuation rules, was ambiguously (in Latin?) either “Don’t be afraid to kill the king. It’s good.” or “don’t kill the king. To be afraid is good.” Depending who you believe, there are suggestions a red hot poker, or a funnel and boiling lead were involved.
Mel Brooks put it simpler. “It’s good to be king.”
Yeah in the alternative reality where he did get comeuppance for it, it would not have been a formal trial, it would have been a group of barons (possibly with an alternative heir to the throne) grabbing him (probably on the battlefield) and throwing him in the tower, followed by a quiet ignoble death. As with Henry VI:
They could have of done the same thing Cromwell did for Charles I, but as others have pointed out there was nothing particularly legal about that, and everyone involved was prosecuted when the monarchy was restored. It’s probably anachronistic to suggest people would have thought that way in Henry II’s era (though Simon De Montfort was taking things in that direction during the first Baron’s war):
It did occur to me that the UK today is a party to the International Criminal Court, who are able to prosecute heads of state. But while some of his actions as king might be of interest to the ICC (what we now call “crimes of aggression” were pretty common back then), a single murder wouldn’t really fall under their jurisdiction. Even if it is in a cathedral.