A scorpion and a frog meet on the banks of a river. The scorpion asks the frog to carry him across on its back. The frog is hesitant, but the scorpion argues that if it were to sting the frog, they would both drown. The frog agrees and starts to swim, but halfway across the scorpion stings it. As the frog feels its life ebbing away, it asks the scorpion why it betrayed its trust. The scorpion replies that it is simply in its nature.
The moral of the story is that you cannot change someone’s nature, no matter how much you trust them.