The story goes back to Aesop. A frog sitting by the riverbank is approached by a scorpion, who asks him to ferry him across. The frog hesitates: “But you’ll sting me and I’ll die.” The scorpion asks, “Where is the reason in that? If I stung you, we’d both die.” Being a reasonable creature, the frog agrees and lets the scorpion hop on. In the middle of the river, the frog feels a horrible pain as he is injected with the scorpion venom. As he feels his body shutting down, he asks: “Why did you do this thing? Now we’ll both die.” I don’t know if scorpions can shrug, but let us say this one can. His last words are: “I can’t help it: It’s my nature.”
Or you can hear Orson Welles tell the same tale in his film Mister Arkadin (1955):
Now what’s the moral of this story insofar as you and I are concerned? Let’s say the scorpion has a shock of bright orange hair. He’s been around for a long time, so we have some notion of how he behaves. Knowing that, why have we allowed that scorpion on our backs?