Although Ney was far from the only Restoration figure to rally to Napoleon during the Hundred Days, there were circumstances that were particular to Ney that singled him out for execution. Unlike his contemporaries, Ney explicitly promised King Louis XVIII when he left Paris at the start of the Hundred Days that he would bring Napoleon back in a cage and instead at Auxerre, Ney turned over his entire army to Napoleon. This not only made Ney's actions a political betrayal, but elevated it to a personal one as well. Other officials that rallied to Napoleon during the Hundred Days might have earlier sworn loyalty to the Bourbons like Soult, but their engagement with the Bourbons in the First Restoration was somewhat limited. Others Hundred Days officials like Davout or Carnot never really took steps to ingratiate themselves to the Bourbons, so their actions did not carry the same opprobrium as did Ney's.
For Bourbon royalists, especially the circles that surrounded the Duchesse d'Angouleme (the surviving daughter of Louis XVI and Marie Antoinette), Ney's defection was emblematic of the systematic rot inside France. Ney's actions at Auxerre made him the most prominent turncloak of the Hundred Days, and thus there was strong pressure to make an example of him. While other Napoleonic marshals like Davout and Soult went back to their old master, none had defected with a whole army under their command. While the ultraroyalists could stomach the stripping of titles and exile of figures like Davout, Ney's actions went beyond rallying to Napoleon after the fact and suggested he planned to actively undermine the King whom he swore allegiance. The d'Angouleme circle put pressure on Louis XVIII to clean house after the second Restoration, and Louis XVIII relented and issued an ordonnance du roi of 57 names to be brought up on charges of treason or exiled from France, Ney's name was listed among the 19 individuals who were to be court-martialed. But the court-martials and presumed capital sentences was quite controversial, even among those French Napoleonic figures who did not turn their cloaks during the Hundred Days. The government's first choice for the presiding judge, Marshal de Mouncey, refused on the grounds that:
Shall 25 years of my glorious labors be sullied in a single day? Shall my locks, bleached under the helmet, be only proofs of my shame? No, Sire! It shall not be said that the elder of the marshals of France contributed to the misfortunes of his country. My life, my fortune, all that I possess or enjoy is at the service of my king and country; but my honor is exclusively my own, and no human power can ravish it from me. If my name is to be the only heritage left to my children, at least let it not be disgraced.
Massena, who pleaded infirmity during the Hundred Days to pledge to either side, likewise refused to serve on the court martial on the grounds his wartime disputes with Ney made him impartial. The strong sentiment in the Restoration military for clemency led to a trial in the Chamber of Peers, which was dominated by emigres.
During this trial, current Minister of War Marshal Gouvion St. Cyr appealed for clemency and so to did several of Ney's colleagues who had found themselves a new role in the Bourbon military. Giving into the revanchist sentiments of d'Angouleme threatened to destroy the fragile political order of the Restoration, which tried to knit together the Napoleonic and Ancien Regime's two aristocracies. Ney's execution also met with wider disapproval in other European circles. In Britain, Lord Holland championed clemency for Ney because it was a violation of Convention of Paris, which was an armistice that promised no political retributions in exchange for peace and the surrender of Paris. Politics were at play here too, as Lord Holland used Ney's execution to attack the rising political star of Wellington by asserting that the British general was seeking revenge upon an enemy he could not defeat in the field. The resurgent British radical press also saw the Bourbon executions as further evidence that the existing political system was rotten and needed radical reform.
Ney did not do himself any favors in this period. Although Davout had negotiated a general amnesty during the transition between the Hundred Days and the second Restoration, a number of Napoleon's lieutenants realized that it would be advisable to leave the country soon. Fouché issued passports to individuals who were on the ordonnance du roi he was drawing up for court martials and exile. Grouchy, for instance, was slated for a court martial, bu was able to use Fouché's assistance to escape to the United States. Ney, rather foolishly, did not correctly read the political situation and insisted he could prove his innocence and his military record would absolve him.
While Ney was wrong on the first count, his military record and composure during his execution managed to salvage his reputation posthumously. The Chamber of Peers, which had been stuffed with a number of Ultras and emigres, overwhelmingly voted for his guilt. The execution of Ney became a rallying cry for Bonapartist sentiments, but sympathy for the Marshal was not limited to them. For the post-Napoleonic France, memories of Napoleonic gloire were quite powerful and multiple parties tried to lay claim to the memory of Napoleon's victories. The Duchess d'Angouleme reportedly wept with guilt when one of the popular accounts of Ney's actions during the 1812 retreat made its way to her. Proscribed officers soon made their way back to France and pardons; Grouchy returned in 1821 with a full restoration of titles and pre-Hundred Days rank and Soult returned the year prior and began an ascent up the ladder of French government to become Minister of War and Prime Minister as well as the French ambassador for Queen Victoria's coronation.
All told the window for political recriminations in the second Restoration was quite brief and a number of French officials appreciated this. Unfortunately for Ney, the window was wide enough for him to be caught up in a net that he stubbornly refused to see was coming.
Sources
Dwyer, Philip G. Citizen Emperor: Napoleon in Power.New Haven : Yale University Press, 2013.
Millar, Stephan. " 'Pour encourager les autres': The Trial and Execution of Marshal Michel Ney." Napoleon-Series, http://www.napoleon-series.org/research/biographies/marshals/c_neyexecution.html .
Muir, Rory. Wellington. Volume 2: Waterloo and the fortunes of peace : 1814 - 1852. New Haven: Yale Univ. Press, 2015.
Price, Munro. Napoleon: The End of Glory. New York: Oxford University Press, 2014.