Follow TV Tropes
Many of the current entries don't seem closely on-topic here. To me, this trope sounds specifically about people who are Just Evil *and* there's some hint that this is the case. A long-running TV show or novel series where people alternately help & betray each other back and forth isn't a good fit; sure, it's poetic that yesterday's allies stabbed you in the back today, but that's just politics. It's not this trope.
What's the difference between this and Ungrateful Bastard?
The focus of the story, I think. Ungrateful Bastard focuses on what a jerk the ungrateful person is, this trope is when the Aesop is more "Well, what did the hero expect"? It's more on the cynical end of the Sliding Scale of Idealism vs. Cynicism; the moral of a story where Superman saves Lex Luthor and Luthor immediately betrays him would never be "Supes should have left Lex to die", because that's not what Superman stories are about.
Another Version of the Farmer and the Viper
After the farmer takes home the frozen viper, it thaws and attacks the farmer and his children. The farmer get an axe and kills the viper.
Moral: Those who repay good with evil may expect their neighbors pity to wear out.
This is "The Countryman and the Snake" from the book 'The Fables of Aesop' that was printed for the Book-of-the-month club.
The version here is a lot more family freindly than the version in this troupe.
Community Showcase More
How well does it match the trope?