Its subject matter was really no darker than other Chaplin ones - he always dealt pretty frankly with poverty and the general horribleness of the world - but its attitude was different.
Even The Great Dictator had this giddy spirit of defiance. The speech at the end was a call to arms, it was stirring. The Little Tramp was all about survival. He always kept going.
The Monsieur Verdoux outlook is more like polite resignation.
It's sad.