The Scorpion and the Frog
(Info)
The scorpion asked the frog to carry him across the river, and the frog asked, "How do I know you won't sting me?" The scorpion replied, "Why would I do that? Then we'd both drown." So the frog agreed. Halfway across the river, the scorpion stung the frog. The frog asked him why he did it since now they would both die, and the scorpion replied, "It's my nature."