A farmer finds a viper, freezing in the snow. Taking pity on it, he picks it up and places it within his
coat. The viper, revived by the warmth, bites his rescuer, who dies realizing that it is his own fault. In
the Greek and Latin fables, the farmer dies reproaching himself “for pitying a scoundrel,” while in the
version by Phaedrus the snake says that he bit his benefactor “to teach the lesson not to expect a
reward from the wicked.”
The moral, kindness to evil will be met by betrayal; the source of the idiom, ‘to nourish a viper in one’s bosom‘.
more info, The farmer and the Snake https://read.gov/aesop/094.html
Wiki : https://en.wikipedia.org/wiki/The_Farmer_and_the_Viper