The Scorpion and the Frog

The Scorpion and the Frog, from Aesop's Fables

A scorpion and a frog meet on the bank of a stream and the scorpion asks the frog to carry him across on its back. The frog asks, "How do I know you won't sting me?" The scorpion says, "Because if I do, I will die too."

The frog is satisfied, and they set out, but in midstream, the scorpion stings the frog. The frog feels the onset of paralysis and starts to sink, knowing they both will drown, but has just enough time to gasp "Why?"

Replies the scorpion: "Its my nature..."

Never expect anyone to act contrary to their nature.