The Farmer and the Snake
Walking in his field on a freezing winter day, a merciful farmer found a snake about to freeze on the roadside. Pitying the snake, the farmer picked it up and placed it inside his coat against his chest to warm it up. Revived and warmed by the farmer's body heat, the snake followed its nature and bit the farmer with a deadly strike. As the poison spread rapidly through the farmer's body, the merciful man fell to the ground breathing his last breath.
The farmer died regretfully, saying "I pay with my life for pitying an ungrateful creature whose nature is evil." Ungrateful beings whose nature is evil respond even to great acts of kindness with evil. Mercy shown to malicious creatures prepares the undoing of the one who shows mercy. It is not wise to show mercy to something poisonous by nature.
Kindness is not done to an ungrateful creature. Nurture a crow and it will peck out your eyes. Mercy brings trouble. The friendship of the venomous poisons.
One who cannot change their nature cannot be trusted. The reward of the ungrateful is evil. Mercy to the wicked is cruelty to the good.