In 1793, the French King Louis XVI was guillotined by the French Revolutionary Tribunal. A relatively quick, and painless death. This was quite a contrast to the death of the menial who had tried to assassinate his father, Louis XV.

https://en.wikipedia.org/wiki/Robert-Fran%C3%A7ois_Damiens

"Fetched from his prison cell on the morning of 28 March 1757, Damiens allegedly said "La journée sera rude" ("The day will be hard").[9] He was first subjected to a torture in which his legs were painfully compressed by devices called "boots".[10][11] He was then tortured with red-hot pincers; the hand with which he had held the knife during the attempted assassination was burned using sulphur; molten wax, molten lead, and boiling oil were poured into his wounds.[1] He was then remanded to the royal executioner Charles-Henri Sanson (who would ironically later go on to execute King Louis XVI) who, after emasculating Damiens, harnessed horses to his arms and legs to be dismembered. But Damiens's limbs did not separate easily: the officiants ordered Sanson to cut Damiens's tendons, and once that was done the horses were able to perform the dismemberment.[10][11][12] Once Damiens was dismembered, to the applause of the crowd, his reportedly still-living torso was burnt at the stake.[13][page needed] (Some accounts say he died when his last remaining arm was removed.)[10][11]

Damiens’s final words are uncertain. Some sources[14] attribute to him "O death, why art thou so long in coming?"; others[15] claim Damiens' last words consisted mainly of various effusions for mercy from God."

Now, it's true, that some of the French aristocrats were tortured and killed by the mob during the French Revolution, but, these were extra-legal procedures, and rather crude, rather than public spectacles of legal torture. So, let's suppose that Louis the XVI, King of France, had been executed by exactly the same extensive public tortures employed on the man who attempted to assassinate his father, Louis XV. What effects will this have, exactly?

Well, I think it might give the other crowned heads of Europe a bit of a pause, actually. Like, "Do we really want to mess with these French lunatics, at all?" Possibly, they would have thought twice about attempting to invade France, given this public demonstration. I also think it might have made the reestablishment of the monarchy in France a literal impossibility. After all, who would want the job, given the risks? It probably would have led to comparable public torture executions of leaders like Danton, Robespierre and maybe even Napoleon, once he started losing.

Would such a public demonstration of legal torture and brutality have reduced support for the French Revolutionaries, or would it have increased it?

It should be borne in mind that in Ireland, Irish patriots were being publicly drawn and quartered by the British until well into the nineteenth century. But, the social implications of this are rather different than the consequences of torturing Kings in public, aren't they? Kings represent a massive public and social investment, far more than the average person. And, publicly torturing and humiliating Kings tends to undermine all the social institutions they may be associated with, doesn't it? So, effectively, is it simply a social impossibility to torture publicly even discredited Kings, because society has such a great investment in them, even after they and their regimes have failed completely?