The mood of "The Man That Corrupted Hadleyburg" is a dark one. Twain expresses sincere pessimism about human nature in his story—it is a gruesome thing to watch a well-regarded town destroy itself from the inside.
Consider, as an example, the bleak final note to the citizens of Hadleyburg that the “stranger” leaves for Reverend Burgess to read at the end of the disastrous assembly in which the town's nineteen prominent citizens reveal themselves as frauds:
I passed through your town at a certain time, and received a deep offense which I had not earned. Any other man would have been content to kill one or two of you and call it square, but to me that would have been a trivial revenge, and inadequate, for the dead do not suffer. Besides I could not kill you all—and, anyway, made as I am, even that would not have satisfied me. I wanted to damage every man in the place, and every woman—and not in their bodies or in their estate, but in their vanity—the place where feeble and foolish people are most vulnerable.
Although "The Man That Corrupted Hadleyburg" contains no shortage of humor, the circumstances that afflict Hadleyburg remain as grim as the stranger explains above: the sack of gold that the stranger sends into the town is an existential threat to its virtue.
