Mark Twain had a poor opinion of human nature. He usually expressed this opinion humorously, but he became increasingly disenchanted with humanity as he aged. Here is a famous quote which exemplifies his thinking on the subject:
All that I care to know is that a man is a human being; that is enough for me; he can't be any worse.
"The Man That Corrupted Hadleyburg" is obviously based on the thesis that there is good and bad in everybody, regardless of how good they may think themselves to be and how good they may try to be. The story shows plainly enough that: "There's a little bit of larceny in everybody." "Every man has his price." "Money talks."
This negative appraisal of humanity, both male and female, may have been fairly new in Mark Twain's time, but a great many authors have made it a subject of their fiction. Both Edgar Allan Poe and Nathaniel Hawthorne used it somewhat earlier in revealing the "dark side" of human nature. Robert Louis Stevenson made it the theme of his "The Strange Case of Dr. Jekyll and Mr. Hyde." Ambrose Bierce expressed a really jaundiced opinion of human nature, especially in The Devil's Dictionary. The Sherlock Holmes stories of Sir Arthur Conan Doyle frequently expose the skeletons in the cupboards of supposedly respectable people, even of the very highest social rank. Guy De Maupassant's short stories are largely about human shortcomings.
Here are a few other quotes from Mark Twain:
Barring that natural expression of villainy which we all have, the man looked honest enough.
If you pick up a starving dog and make him prosperous, he will not bite you. That is the principal difference between a dog and a man.
Of all the creatures that were made he [man] is the most detestable. Of the entire brood he is the only one--the solitary one--that possesses malice.