Fable symbolising the pitiless dark angel of human nature

A scorpion asks a frog to ferry it across a stream. The frog at first refuses, saying that it fears the scorpion will sting it. The scorpion assures the frog it will do no such thing. After all, it says, we will both perish if I stimg you. The frog consents, and halfway across the stream the scorpion stings the frog. Why did you do that, the frog asks as they both sink beneath the surface. It is my nature, the scorpion explains.