Yes I must agree that only in Republican France, which had murdered the previous Bourbon, (apparently to make way for another, albeit more enlightened, autocrat) was he seen as legitimate, and let us be clear he was allowed to become First Consul only because the Bonapartes... ahem... removed the Directory. To the other powers of Europe the line of succession had been broken, and a man without link to the royal family had taken their place on the throne.
Interesting that you use the term 'murder' to describe the executions of Bourbons, but not the judicial murders of Ney and Labedoyere, along with the outright murders of such as Brune...
Napoleon became First Consul because he was recruited for the coup, he was not the originator of it. The conspirators needed a competent general officer to lead the coup and Napoleon was not their first choice. That they used an officer much more competent than they was fortunate for France as Napoleon did restore law and order and returned France to a new normalcy. Napoleon first asked to become a Director, but was told he was too young, which he thought nonsense. And he found out that one of the Directors, Barras, was contemplating returning the Bourbons to power, which no one wanted.
Look at the Jacobite succession crisis in Britain. James II was a catholic, and so he was ousted by the government and his son in law William of Orange offered the throne, which passed back to the last officially Protestant Stuart monarch, Anne. When she died the Elector of Hanover got the job, much to the outrage of James II's supporters who proceeded to launch 4 large scale rebellions between 1689 and 1745 and numberless plots. To the Jacobites they were merely the supporters of the rightful king with the strongest link, albeit legally disbarred on religious grounds, whereas the new House of Hanover branded them "Pretenders", had they won the throne, the Hanoverians would have branded them usurpers, a name which the Jacobites had already branded George I with.
You left out Charles I-would you label that an execution or murder? It was clearly done because he lost to the Parliamentarians.
Something to some is not the same to others.
There is no propoganda about it. Legally and some have been playing with legal niceties to defend the death of D'Enghien while apparently condemning Ney's and labedoyere's executions, legally he was defined as a usurper in Europe because he took the throne.
There most certainly a great aspect of propaganda to the issued of the period, especially in regard to Napoleon. There are no 'legal niceties' regarding d'Enghien-he dabbled in treason, was in the pay of a foreign power at war with France, and planned to fight against her. That's guilt, pure and simple.