You can debate Napoleon pro and con, but if you’re asking why he was villified, keep in mind that “History is written by the winners” and propaganda tends to go into overdrive during wars. Napoleon not only conquered just about all of Europe (fighting England about the whole time), he then was doing pretty well on his surprise comeback tour. So naturally at the time, especially among English speakers, he was public enemy number one, even after the war was over, given his first comeback
There’s also the issue that he was to some degree a symbol of the French Revolution, and Kings and Dukes and the like didn’t really want to see those kinds of royal head-chopping ideas floating about Europe.
And CalMeacham, maybe getting off-topic, but if you think War and Peace makes Napoleon out to be an ogre, you should read Resurrection, about a travesty of justice in Russia under the Czars, and ask yourself if the Czars were any better? (actually the book isn’t that great as literature, so maybe don’t spend the time to read it. But do ask yourself if the Czar was any better). I don’t think all the sailors in C.S. Forester’s novels were really well-treated volunteers, whose trips to India and other places involved meeting with fellow-citizens grateful for the chance to be part of the Free and Fair Association of Independent States Aligned with Britain for the Common Good.
Personally, I though Tolstoy did make Napoleon out to be petty, small-minded and egotistical, but Tolstoy showed all of his characters’ pettiness and egotism (and occaisonal more redeeming qualities, too). And he made strong statements that no one person was really in control of events – that you couldn’t blame Napoleon more than the ones actually pulling triggers, doing the looting, etc.