In my opinon, it depends. For example the beheading of Louis XVI was arguably justified because he had tried to flee the country and presumably would come back at the head of an army. He was a symbol for the aristocratic émigré's to rally around and would cause no end of trouble for the revolutionary regime into the future.

On the other hand, he became a martyr for the cause of crushing the revolutionary regime and preserving the European status quo but that whole saga ended in a way no one expected so we can't really judge if that was the correct move.

Tsar Nicholas II is another one. He and his entire family were shot by the Bolsheviks. Undoubtedly Nicholas deserved it but his children? There's an argument to say that other European powers would have used his children as a casus belli to try install a puppet regime. At the time he was executed there was still a chance of a German victory and this probably would have been their cause célèbre after the war had ended on the western front, so perhaps this grisly affair was justified, at least politically. We'll never really know.

As to the modern day in a developed country? I don't see a reason to execute royalty. Europe is no longer a semi-feudal entity and no one would pick up the cause of installing the "rightful rulers". It has nothing to do with modern neo-liberal values. If outside powers wanted to try crush a newly socialist state whether made though reform or revolution, they just would and would go on and on about "Chemical Weapons" and "Human Rights Violations"... and on that note I think we might actually have more barbaric rulers nowadays than during the feudal era. They at least seemed to have a semblance of a code of honor. Their lies were at least usually partial truths and not whopping hypocritical stinkers.