Its actually a scorpion asking a frog to carry him across the river. The frog agrees, on the condition that the scorpion won't sting him. However when they cross, the scorpion stings the frog. The frog says "you idiot, now we are both going to die. Why did you do that" To which the scorpion replies, "because I'm a scorpion"
Isn't that in Kill Bill?