He may have had a terrible personality an may have killed many, many people.
But in the eyes of the French back then he was truely a hero, a conqueror.

And don't forget the Code Napoléon. He knew military victories were only temporarily, but a legal victory would last! And he was right, because except for the common law lands, every single other nation has had a civil code that was either litteraly the Code Napoléon or a derivate of it.
Of course he didn't right the code himself, but he did attend half the meetings.
Not that he improved it much, because Napoleon was Coriscan provincial boy, with conservative provincial views. So in some ways it was even more backward than the "backward" local rules that existed everywhere, it replaced.

For example Napoleon made the women totally unable. Because of this they had to accompagnied by or would need written permission of their husbands or fathers every time they went to buy a bread.
And he only kept divorce legal because he wanted to get rid of his own wife.

Anyway, he did some good things too. Their even is a Western European nation who still uses the Code Napoléon (in name, it's content has been adapted by special laws) and it ain't France.

This whole code thing was pure propaganda for Napoleon. Before him you had different law every 5 miles, now you had the same law in most parts of the world. This made him a hero with the upper classes and his military victories made him popular with the lower classes.