A big reason is that he never did directly cause much of it, though he did partake in it. Cases like the Invasion of Russia in 1812 and the guerilla war in Spain can be directly attributed to Napoleon, but otherwise most wars were actually started by the numerous coalitions against him as the members ebbed and flowed, and did their fortunes.

But besides the fact that the vast majority of the wars he fought (even, to some extent, Russia and Spain) were defensive wars, he was a great liberaliser and instrumental in the codification of equal rights, meritocracy, religious freedoms and the end of the vestiges of feudalism in Europe and abroad. His Napoleonic Code was adapted by various means and interpretations as far as the Middle East and laid the ground work for future advancements in progressive changes.

Basically, a mix of moral neutrality and political advancements by Napoleon allowed the zeitgeist of his being beloved or mere neutrally to come into being, and they're not wrong either. While human rights and equality still had a long time to progress after him, for example in the realm of enfranchisement, he bricked the walls and continued the work of the Revolution in destroying the old ways of Europe. It was no longer a time of kings and the aristocracy dictating to the world, it was a transition between that and the modern day.

The reputation of Napoleon had two factors working in its favor despite his somewhat questionable habit of military conquests and Napoleon's ignominious end.

One of Napoleon's central claims to fame was his reputation as a state-builder. In France, he instituted a degree of reforms like the Napoleonic Code and established institutions like the Légion d'honneur that would endure both Napoleon and Bonapartism. Additionally, this reformism did extend to areas conquered by French arms. Napoleon had inherited, and during his first Italian campaign pioneered, the French Revolution's tendency to export revolution. Within both satellite states and areas annexed by France, new elites emerged as servitors to the French Empire and political order and many of these elites continued to ply their trade after Waterloo. Napoleon's reforms were also not exactly liberal or progressive by the standard of their time. One of Napoleon's axioms for constitutions was that they should be short and vague. Nor were many of these reforms immediately successful either. Napoleon's Concordat with the Catholic Church ushered in a good half a century of strained religious policies and neither the Franc nor the Banque de France were able to finance Napoleon's escalating number of wars. As French finances fell into a black hole, Napoleon shifted the French defense burden onto conquests and satellite states, which in turn prompted an expansion and systematizing of tax collection throughout Europe. But despite this mixed record on the ground, hindsight gave Napoleon a reputation as one of the architects of the modern nation-state system. This set him apart from the images of other historical conquerors during the nineteenth century. Although the actual history of both Genghis Khan and Attila the Hun were quite as complex as Napoleon's, nineteenth-century thought tended to regard these two as blood-thirsty barbarian warlords who left nothing for all their destruction (and this inaccurate image casts a long shadow as Matt Groening's writing teams' depiction of Attila and the Great Khan shows). In this respect, Napoleon was in much better company.

The other aspect working in Napoleon's favor was the great man's own historical narrative. Napoleon managed to rise from an obscure provincial backwater to being a master of Europe in less than a decade. He accomplished this, as Napoleonic propaganda was fond of reminding everyone, largely through his own talents and abilities. Even during his own lifetime, many saw this as a remarkable achievement. This meant he was something of a role model for the new world of the nineteenth century characterized by the vast expansion of capital and the transgressive industrial order and mass politics. It was no surprise that among the most enthusiastic collectors of Napoleonic memorabilia in the latter half of the nineteenth century were the new American industrial elite. Napoleon seemed to provide a model for genius that could transcend the boundaries of convention and accomplish the impossible. To be "a Napoleon" in the nineteenth century was not the same as being a Hitler/Nazi in 2016, but rather to be someone with great ambitions that could borer on hubris. Like his state-building, the historical record of Napoleon's talents and genius did have something of a mixed record, but the romantic image of Napoleon struck a deep cord for modern society.

In his study of Napoleon's reputation, R. S. Arnold perceptively noted that while Napoleon was not a man of the people, there are many "Napoleons" among the people. Often times the image of Napoleon is at odds with his actual record; Napoleon did herald the modern era, but, as H.G. Wells tartly put it "he could have been the father of a new era, but instead chose to be the stepson of the old." His achievements though, embellished by a romanticized image of the Emperor, offer the tempting promise of a lone figure who is able to transcend the strictures of his time.