"Aesop's frozen serpent"

This is a reference to the moralistic tale of The Farmer and the Viper, one of Aesop's Fables. A farmer finds a viper frozen in the snow. Pitying it, he picks it up and places it inside his coat to warm it. The viper revives and bites the farmer, inflicting a fatal wound. In an alternate version, the farmer takes the viper home and warms it by the fire, where it awakes and threatens his family and has to be killed with an axe. The moral speaks of kindness betrayed by the ungrateful, and of the unchanging nature of evil.

Aesop's Fables